### 2017

Stenflo, J O

History of Solar Magnetic Fields Since George Ellery Hale Journal Article

In: Space Science Reviews, vol. 210, pp. 5-35, 2017.

@article{2017SSRv..210....5S,

title = {History of Solar Magnetic Fields Since George Ellery Hale},

author = {J O {Stenflo}},

url = {http://adsabs.harvard.edu/abs/2017SSRv..210....5S},

doi = {10.1007/s11214-015-0198-z},

year = {2017},

date = {2017-09-01},

journal = {Space Science Reviews},

volume = {210},

pages = {5-35},

abstract = {As my own work on the Sun's magnetic field started exactly 50 years ago

at Crimea in the USSR, I have been a participant in the field during

nearly half the time span since Hale's discovery in 1908 of magnetic

fields in sunspots. The present historical account is accompanied by

photos from my personal slide collection, which show a number of the

leading personalities who advanced the field in different areas:

measurement techniques, from photographic to photoelectric and imaging

methods in spectro-polarimetry; theoretical foundations of MHD and the

origin of cosmic magnetic fields (birth of dynamo theory); the quest for

increased angular resolution from national projects to international

consortia (for instruments both on ground and in space); introduction of

the Hanle effect in astrophysics and the Second Solar Spectrum as its

playground; small-scale nature of the field, the fundamental resolution

limit, and transcending it by resolution-independent diagnostics. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

at Crimea in the USSR, I have been a participant in the field during

nearly half the time span since Hale's discovery in 1908 of magnetic

fields in sunspots. The present historical account is accompanied by

photos from my personal slide collection, which show a number of the

leading personalities who advanced the field in different areas:

measurement techniques, from photographic to photoelectric and imaging

methods in spectro-polarimetry; theoretical foundations of MHD and the

origin of cosmic magnetic fields (birth of dynamo theory); the quest for

increased angular resolution from national projects to international

consortia (for instruments both on ground and in space); introduction of

the Hanle effect in astrophysics and the Second Solar Spectrum as its

playground; small-scale nature of the field, the fundamental resolution

limit, and transcending it by resolution-independent diagnostics.

Janett, Gioele; Steiner, Oskar; Belluzzi, Luca

Formal Solutions for Polarized Radiative Transfer. II. High-order Methods Journal Article

In: The Astrophysical Journal, vol. 845, no. 2, pp. 104, 2017.

@article{0004-637X-845-2-104,

title = {Formal Solutions for Polarized Radiative Transfer. II. High-order Methods},

author = {Gioele Janett and Oskar Steiner and Luca Belluzzi},

url = {http://www.irsol.usi.ch/wp-content/uploads/2017/09/apj_845_2_104o.pdf

http://stacks.iop.org/0004-637X/845/i=2/a=104},

year = {2017},

date = {2017-08-01},

journal = {The Astrophysical Journal},

volume = {845},

number = {2},

pages = {104},

abstract = {When integrating the radiative transfer equation for polarized light, the necessity of high-order numerical methods is well known. In fact, well-performing high-order formal solvers enable higher accuracy and the use of coarser spatial grids. Aiming to provide a clear comparison between formal solvers, this work presents different high-order numerical schemes and applies the systematic analysis proposed by Janett et al., emphasizing their advantages and drawbacks in terms of order of accuracy, stability, and computational cost.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sampoorna, M; Nagendra, K N; Stenflo, J O

Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution Journal Article

In: Astrophysical Journal, vol. 844, pp. 97, 2017.

@article{2017ApJ...844...97S,

title = {Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution},

author = {M {Sampoorna} and K N {Nagendra} and J O {Stenflo}},

url = {http://adsabs.harvard.edu/abs/2017ApJ...844...97S},

doi = {10.3847/1538-4357/aa7a15},

year = {2017},

date = {2017-08-01},

journal = {Astrophysical Journal},

volume = {844},

pages = {97},

abstract = {Magnetic fields in the solar atmosphere leave their fingerprints in the

polarized spectrum of the Sun via the Hanle and Zeeman effects. While

the Hanle and Zeeman effects dominate, respectively, in the weak and

strong field regimes, both these effects jointly operate in the

intermediate field strength regime. Therefore, it is necessary to solve

the polarized line transfer equation, including the combined influence

of Hanle and Zeeman effects. Furthermore, it is required to take into

account the effects of partial frequency redistribution (PRD) in

scattering when dealing with strong chromospheric lines with broad

damping wings. In this paper, we present a numerical method to solve the

problem of polarized PRD line formation in magnetic fields of arbitrary

strength and orientation. This numerical method is based on the concept

of operator perturbation. For our studies, we consider a two-level atom

model without hyperfine structure and lower-level polarization. We

compare the PRD idealization of angle-averaged Hanle-Zeeman

redistribution matrices with the full treatment of angle-dependent PRD,

to indicate when the idealized treatment is inadequate and what kind of

polarization effects are specific to angle-dependent PRD. Because the

angle-dependent treatment is presently computationally prohibitive when

applied to realistic model atmospheres, we present the computed emergent

Stokes profiles for a range of magnetic fields, with the assumption of

an isothermal one-dimensional medium. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

polarized spectrum of the Sun via the Hanle and Zeeman effects. While

the Hanle and Zeeman effects dominate, respectively, in the weak and

strong field regimes, both these effects jointly operate in the

intermediate field strength regime. Therefore, it is necessary to solve

the polarized line transfer equation, including the combined influence

of Hanle and Zeeman effects. Furthermore, it is required to take into

account the effects of partial frequency redistribution (PRD) in

scattering when dealing with strong chromospheric lines with broad

damping wings. In this paper, we present a numerical method to solve the

problem of polarized PRD line formation in magnetic fields of arbitrary

strength and orientation. This numerical method is based on the concept

of operator perturbation. For our studies, we consider a two-level atom

model without hyperfine structure and lower-level polarization. We

compare the PRD idealization of angle-averaged Hanle-Zeeman

redistribution matrices with the full treatment of angle-dependent PRD,

to indicate when the idealized treatment is inadequate and what kind of

polarization effects are specific to angle-dependent PRD. Because the

angle-dependent treatment is presently computationally prohibitive when

applied to realistic model atmospheres, we present the computed emergent

Stokes profiles for a range of magnetic fields, with the assumption of

an isothermal one-dimensional medium.

Carlin, E. S.; M.Bianda,

Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line Journal Article

In: Astrophysical Journal, vol. 843, pp. 64, 2017.

@article{2017ApJ...843...64C,

title = {Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line},

author = {E. S. Carlin and M.Bianda},

url = {https://arxiv.org/abs/1706.02381},

doi = {10.3847/1538-4357/aa7800},

year = {2017},

date = {2017-07-01},

journal = {Astrophysical Journal},

volume = {843},

pages = {64},

abstract = {Due to the quick evolution of the solar chromosphere, its magnetic field cannot be inferred reliably without accounting for the temporal variations of its polarized light. This has been broadly overlooked in the modeling and interpretation of the polarization, due to technical problems (e.g., lack of temporal resolution or of time-dependent MHD solar models) and/or because many polarization measurements can apparently be explained without dynamics. Here, we show that the temporal evolution is critical for explaining the spectral-line scattering polarization because of its sensitivity to rapidly varying physical quantities and the possibility of signal cancellations and attenuation during extended time integration. For studying the combined effect of time-varying magnetic fields and kinematics, we solved the 1.5D non-LTE problem of the second kind in time-dependent 3D R-MHD solar models and synthesized the Hanle and Zeeman polarization in forward scattering for the chromospheric λ4227 line. We find that the quiet-Sun polarization amplitudes depend on the periodicity and spectral coherence of the signal enhancements produced by kinematics, but that substantially larger linear polarization signals should exist all over the solar disk for short integration times. The spectral morphology of the polarization is discussed as a combination of Hanle, Zeeman, partial redistribution and dynamic effects. We give physical references for observations by degrading and characterizing our slit time series in different spatiotemporal resolutions. The implications of our results for the interpretation of the second solar spectrum and for the investigation of the solar atmospheric heatings are discussed. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Janett, Gioele; Carlin, Edgar S.; Steiner, Oskar; Belluzzi, Luca

Formal Solutions for Polarized Radiative Transfer. I. The DELO Family Journal Article

In: The Astrophysical Journal, vol. 840, no. 2, pp. 107, 2017.

@article{0004-637X-840-2-107b,

title = {Formal Solutions for Polarized Radiative Transfer. I. The DELO Family},

author = {Gioele Janett and Edgar S. Carlin and Oskar Steiner and Luca Belluzzi},

url = {http://www.irsol.usi.ch/wp-content/uploads/2017/09/apj_840_2_107o.pdf

http://stacks.iop.org/0004-637X/840/i=2/a=107

},

year = {2017},

date = {2017-05-01},

journal = {The Astrophysical Journal},

volume = {840},

number = {2},

pages = {107},

abstract = {The discussion regarding the numerical integration of the polarized radiative transfer equation is still open and the comparison between the different numerical schemes proposed by different authors in the past is not fully clear. Aiming at facilitating the comprehension of the advantages and drawbacks of the different formal solvers, this work presents a reference paradigm for their characterization based on the concepts of order of accuracy , stability , and computational cost . Special attention is paid to understand the numerical methods belonging to the Diagonal Element Lambda Operator family, in an attempt to highlight their specificities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Kano, R; Bueno, J Trujillo; Winebarger, A; Auchère, F; Narukage, N; Ishikawa, R; Kobayashi, K; Bando, T; Katsukawa, Y; Kubo, M; Ishikawa, S; Giono, G; Hara, H; Suematsu, Y; Shimizu, T; Sakao, T; Tsuneta, S; Ichimoto, K; Goto, M; Belluzzi, L; Stepán, J; Ramos, A Asensio; Sainz, R Manso; Champey, P; Cirtain, J; Pontieu, B De; Casini, R; Carlsson, M

Discovery of Scattering Polarization in the Hydrogen Ly-α Line of the Solar Disk Radiation Journal Article

In: Astrophysical Journal, Letters, vol. 839, pp. L10, 2017.

@article{2017ApJ...839L..10K,

title = {Discovery of Scattering Polarization in the Hydrogen Ly-α Line of the Solar Disk Radiation},

author = {R {Kano} and J {Trujillo Bueno} and A {Winebarger} and F {Auchère} and N {Narukage} and R {Ishikawa} and K {Kobayashi} and T {Bando} and Y {Katsukawa} and M {Kubo} and S {Ishikawa} and G {Giono} and H {Hara} and Y {Suematsu} and T {Shimizu} and T {Sakao} and S {Tsuneta} and K {Ichimoto} and M {Goto} and L {Belluzzi} and J {Stepán} and A {Asensio Ramos} and R {Manso Sainz} and P {Champey} and J {Cirtain} and B {De Pontieu} and R {Casini} and M {Carlsson}},

url = {http://adsabs.harvard.edu/abs/2017ApJ...839L..10K},

doi = {10.3847/2041-8213/aa697f},

year = {2017},

date = {2017-05-01},

journal = {Astrophysical Journal, Letters},

volume = {839},

pages = {L10},

abstract = {There is a thin transition region (TR) in the solar atmosphere where the

temperature rises from 10,000 K in the chromosphere to millions of

degrees in the corona. Little is known about the mechanisms that

dominate this enigmatic region other than the magnetic field plays a key

role. The magnetism of the TR can only be detected by polarimetric

measurements of a few ultraviolet (UV) spectral lines, the Ly$alpha$

line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV

spectrum) being of particular interest given its sensitivity to the

Hanle effect (the magnetic-field-induced modification of the scattering

line polarization). We report the discovery of linear polarization

produced by scattering processes in the Lyα line, obtained with

the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket

experiment. The Stokes profiles observed by CLASP in quiet regions of

the solar disk show that the Q/I and U/I linear polarization signals are

of the order of 0.1% in the line core and up to a few percent in the

nearby wings, and that both have conspicuous spatial variations with

scales of tilde10 arcsec. These observations help constrain theoretical

models of the chromosphere-corona TR and extrapolations of the

magnetic field from photospheric magnetograms. In fact, the observed

spatial variation from disk to limb of polarization at the line core and

wings already challenge the predictions from three-dimensional

magnetohydrodynamical models of the upper solar chromosphere. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

temperature rises from 10,000 K in the chromosphere to millions of

degrees in the corona. Little is known about the mechanisms that

dominate this enigmatic region other than the magnetic field plays a key

role. The magnetism of the TR can only be detected by polarimetric

measurements of a few ultraviolet (UV) spectral lines, the Ly$alpha$

line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV

spectrum) being of particular interest given its sensitivity to the

Hanle effect (the magnetic-field-induced modification of the scattering

line polarization). We report the discovery of linear polarization

produced by scattering processes in the Lyα line, obtained with

the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket

experiment. The Stokes profiles observed by CLASP in quiet regions of

the solar disk show that the Q/I and U/I linear polarization signals are

of the order of 0.1% in the line core and up to a few percent in the

nearby wings, and that both have conspicuous spatial variations with

scales of tilde10 arcsec. These observations help constrain theoretical

models of the chromosphere-corona TR and extrapolations of the

magnetic field from photospheric magnetograms. In fact, the observed

spatial variation from disk to limb of polarization at the line core and

wings already challenge the predictions from three-dimensional

magnetohydrodynamical models of the upper solar chromosphere.

Giono, G; Ishikawa, R; Narukage, N; Kano, R; Katsukawa, Y; Kubo, M; Ishikawa, S; Bando, T; Hara, H; Suematsu, Y; Winebarger, A; Kobayashi, K; Auchère, F; Bueno, J Trujillo; Tsuneta, S; Shimizu, T; Sakao, T; Cirtain, J; Champey, P; Ramos, A Asensio; v Stv epán, J; Belluzzi, L; Sainz, R Manso; Pontieu, B De; Ichimoto, K; Carlsson, M; Casini, R; Goto, M

In: Solar Physics, vol. 292, pp. 57, 2017.

@article{2017SoPh..292...57G,

title = {Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration},

author = {G {Giono} and R {Ishikawa} and N {Narukage} and R {Kano} and Y {Katsukawa} and M {Kubo} and S {Ishikawa} and T {Bando} and H {Hara} and Y {Suematsu} and A {Winebarger} and K {Kobayashi} and F {Auchère} and J {Trujillo Bueno} and S {Tsuneta} and T {Shimizu} and T {Sakao} and J {Cirtain} and P {Champey} and A {Asensio Ramos} and J {{v S}t{v e}pán} and L {Belluzzi} and R {Manso Sainz} and B {De Pontieu} and K {Ichimoto} and M {Carlsson} and R {Casini} and M {Goto}},

url = {http://adsabs.harvard.edu/abs/2017SoPh..292...57G},

doi = {10.1007/s11207-017-1062-y},

year = {2017},

date = {2017-04-01},

journal = {Solar Physics},

volume = {292},

pages = {57},

abstract = {The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket

instrument designed to measure for the first time the linear

polarization of the hydrogen Lyman-$$$alpha$$$ line (121.6 nm). The

instrument was successfully launched on 3 September 2015 and

observations were conducted at the solar disc center and close to the

limb during the five-minutes flight. In this article, the disc center

observations are used to provide an in-flight calibration of the

instrument spurious polarization. The derived in-flight spurious

polarization is consistent with the spurious polarization levels

determined during the pre-flight calibration and a statistical analysis

of the polarization fluctuations from solar origin is conducted to

ensure a 0.014% precision on the spurious polarization. The combination

of the pre-flight and the in-flight polarization calibrations provides a

complete picture of the instrument response matrix, and a proper error

transfer method is used to confirm the achieved polarization accuracy.

As a result, the unprecedented 0.1% polarization accuracy of the

instrument in the vacuum ultraviolet is ensured by the polarization

calibration. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

instrument designed to measure for the first time the linear

polarization of the hydrogen Lyman-$$$alpha$$$ line (121.6 nm). The

instrument was successfully launched on 3 September 2015 and

observations were conducted at the solar disc center and close to the

limb during the five-minutes flight. In this article, the disc center

observations are used to provide an in-flight calibration of the

instrument spurious polarization. The derived in-flight spurious

polarization is consistent with the spurious polarization levels

determined during the pre-flight calibration and a statistical analysis

of the polarization fluctuations from solar origin is conducted to

ensure a 0.014% precision on the spurious polarization. The combination

of the pre-flight and the in-flight polarization calibrations provides a

complete picture of the instrument response matrix, and a proper error

transfer method is used to confirm the achieved polarization accuracy.

As a result, the unprecedented 0.1% polarization accuracy of the

instrument in the vacuum ultraviolet is ensured by the polarization

calibration.

Jafarzadeh, S; Solanki, S K; Stangalini, M; Steiner, O; Cameron, R H; Danilovic, S

High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI Journal Article

In: Astrophysical Journal, Supplement, vol. 229, pp. 10, 2017.

@article{2017ApJS..229...10J,

title = {High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI},

author = {S {Jafarzadeh} and S K {Solanki} and M {Stangalini} and O {Steiner} and R H {Cameron} and S {Danilovic}},

doi = {10.3847/1538-4365/229/1/10},

year = {2017},

date = {2017-04-01},

journal = {Astrophysical Journal, Supplement},

volume = {229},

pages = {10},

abstract = {We characterize waves in small magnetic elements and investigate their

propagation in the lower solar atmosphere from observations at high

spatial and temporal resolution. We use the wavelet transform to analyze

oscillations of both horizontal displacement and intensity in magnetic

bright points found in the 300 nm and the Ca II H 396.8 nm passbands of

the filter imager on board the Sunrise balloon-borne solar observatory.

Phase differences between the oscillations at the two atmospheric layers

corresponding to the two passbands reveal upward propagating waves at

high frequencies (up to 30 mHz). Weak signatures of standing as well as

downward propagating waves are also obtained. Both compressible and

incompressible (kink) waves are found in the small-scale magnetic

features. The two types of waves have different, though overlapping,

period distributions. Two independent estimates give a height difference

of approximately 450 plusmn 100 km between the two atmospheric layers

sampled by the employed spectral bands. This value, together with the

determined short travel times of the transverse and longitudinal waves

provide us with phase speeds of 29 plusmn 2 km s$^-1$ and 31

plusmn 2 km s$^-1$, respectively. We speculate that these

phase speeds may not reflect the true propagation speeds of the waves.

Thus, effects such as the refraction of fast longitudinal waves may

contribute to an overestimate of the phase speed. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

propagation in the lower solar atmosphere from observations at high

spatial and temporal resolution. We use the wavelet transform to analyze

oscillations of both horizontal displacement and intensity in magnetic

bright points found in the 300 nm and the Ca II H 396.8 nm passbands of

the filter imager on board the Sunrise balloon-borne solar observatory.

Phase differences between the oscillations at the two atmospheric layers

corresponding to the two passbands reveal upward propagating waves at

high frequencies (up to 30 mHz). Weak signatures of standing as well as

downward propagating waves are also obtained. Both compressible and

incompressible (kink) waves are found in the small-scale magnetic

features. The two types of waves have different, though overlapping,

period distributions. Two independent estimates give a height difference

of approximately 450 plusmn 100 km between the two atmospheric layers

sampled by the employed spectral bands. This value, together with the

determined short travel times of the transverse and longitudinal waves

provide us with phase speeds of 29 plusmn 2 km s$^-1$ and 31

plusmn 2 km s$^-1$, respectively. We speculate that these

phase speeds may not reflect the true propagation speeds of the waves.

Thus, effects such as the refraction of fast longitudinal waves may

contribute to an overestimate of the phase speed.

Ballester, E. A.; Belluzzi, L.; Trujillo Bueno, J.

The Transfer of Resonance Line Polarization with Partial Frequency Redistribution in the General Hanlen-Zeeman Regime Journal Article

In: Astrophysical Journal, vol. 836, pp. 6, 2017.

@article{2017ApJ...836....6B,

title = {The Transfer of Resonance Line Polarization with Partial Frequency Redistribution in the General Hanlen-Zeeman Regime},

author = {Ballester, E.A. and Belluzzi, L. and Trujillo Bueno, J.},

url = {https://arxiv.org/pdf/1609.05723},

doi = {10.3847/1538-4357/836/1/6},

year = {2017},

date = {2017-02-01},

journal = {Astrophysical Journal},

volume = {836},

pages = {6},

abstract = {The spectral line polarization encodes a wealth of information about the

thermal and magnetic properties of the solar atmosphere. Modeling the

Stokes profiles of strong resonance lines is, however, a complex problem

both from a theoretical and computational point of view, especially when

partial frequency redistribution (PRD) effects need to be taken into

account. In this work, we consider a two-level atom in the presence of

magnetic fields of arbitrary intensity (HanlendashZeeman regime) and

orientation, both deterministic and micro-structured. Working within the

framework of a rigorous PRD theoretical approach, we have developed a

numerical code that solves the full non-LTE radiative transfer problem

for polarized radiation, in one-dimensional models of the solar

atmosphere, accounting for the combined action of the Hanle and Zeeman

effects, as well as for PRD phenomena. After briefly discussing the

relevant equations, we describe the iterative method of solution of the

problem and the numerical tools that we have developed and implemented.

We finally present some illustrative applications to two resonance lines

that form at different heights in the solar atmosphere, and provide a

detailed physical interpretation of the calculated Stokes profiles. We

find that magneto-optical effects have a strong impact on the linear

polarization signals that PRD effects produce in the wings of strong

resonance lines. We also show that the weak-field approximation has to

be used with caution when PRD effects are considered. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

thermal and magnetic properties of the solar atmosphere. Modeling the

Stokes profiles of strong resonance lines is, however, a complex problem

both from a theoretical and computational point of view, especially when

partial frequency redistribution (PRD) effects need to be taken into

account. In this work, we consider a two-level atom in the presence of

magnetic fields of arbitrary intensity (HanlendashZeeman regime) and

orientation, both deterministic and micro-structured. Working within the

framework of a rigorous PRD theoretical approach, we have developed a

numerical code that solves the full non-LTE radiative transfer problem

for polarized radiation, in one-dimensional models of the solar

atmosphere, accounting for the combined action of the Hanle and Zeeman

effects, as well as for PRD phenomena. After briefly discussing the

relevant equations, we describe the iterative method of solution of the

problem and the numerical tools that we have developed and implemented.

We finally present some illustrative applications to two resonance lines

that form at different heights in the solar atmosphere, and provide a

detailed physical interpretation of the calculated Stokes profiles. We

find that magneto-optical effects have a strong impact on the linear

polarization signals that PRD effects produce in the wings of strong

resonance lines. We also show that the weak-field approximation has to

be used with caution when PRD effects are considered.

Vigeesh, G.; Jackiewicz, J.; Steiner, O.

Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects Journal Article

In: Astrophysical Journal, vol. 835, pp. 148, 2017.

@article{2017ApJ...835..148V,

title = {Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects},

author = {Vigeesh, G. and Jackiewicz, J. and Steiner, O.},

url = {https://arxiv.org/abs/1612.04729},

doi = {10.3847/1538-4357/835/2/148},

year = {2017},

date = {2017-02-01},

journal = {Astrophysical Journal},

volume = {835},

pages = {148},

abstract = {Observations of the solar atmosphere show that internal gravity waves

are generated by overshooting convection, but are suppressed at

locations of magnetic flux, which is thought to be the result of mode

conversion into magnetoacoustic waves. Here, we present a study of the

acoustic-gravity wave spectrum emerging from a realistic,

self-consistent simulation of solar (magneto)convection. A magnetic

field free, hydrodynamic simulation and a magnetohydrodynamic (MHD)

simulation with an initial, vertical, homogeneous field of 50 G flux

density were carried out and compared with each other to highlight the

effect of magnetic fields on the internal gravity wave propagation in

the Sunrsquos atmosphere. We find that the internal gravity waves are

absent or partially reflected back into the lower layers in the presence

of magnetic fields and argue that the suppression is due to the coupling

of internal gravity waves to slow magnetoacoustic waves still within the

high-$beta$ region of the upper photosphere. The conversion to

Alfv'en waves is highly unlikely in our model because there is no

strongly inclined magnetic field present. We argue that the suppression

of internal waves observed within magnetic flux concentrations may also

be due to nonlinear breaking of internal waves due to vortex flows that

are ubiquitously present in the upper photosphere and the chromosphere. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

are generated by overshooting convection, but are suppressed at

locations of magnetic flux, which is thought to be the result of mode

conversion into magnetoacoustic waves. Here, we present a study of the

acoustic-gravity wave spectrum emerging from a realistic,

self-consistent simulation of solar (magneto)convection. A magnetic

field free, hydrodynamic simulation and a magnetohydrodynamic (MHD)

simulation with an initial, vertical, homogeneous field of 50 G flux

density were carried out and compared with each other to highlight the

effect of magnetic fields on the internal gravity wave propagation in

the Sunrsquos atmosphere. We find that the internal gravity waves are

absent or partially reflected back into the lower layers in the presence

of magnetic fields and argue that the suppression is due to the coupling

of internal gravity waves to slow magnetoacoustic waves still within the

high-$beta$ region of the upper photosphere. The conversion to

Alfv'en waves is highly unlikely in our model because there is no

strongly inclined magnetic field present. We argue that the suppression

of internal waves observed within magnetic flux concentrations may also

be due to nonlinear breaking of internal waves due to vortex flows that

are ubiquitously present in the upper photosphere and the chromosphere.

Bueno, J Trujillo; Degl'Innocenti, E Landi; Belluzzi, L

The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry Journal Article

In: Space Science Reviews, vol. 210, pp. 183-226, 2017.

@article{2017SSRv..210..183T,

title = {The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry},

author = {J {Trujillo Bueno} and E {Landi Degl'Innocenti} and L {Belluzzi}},

doi = {10.1007/s11214-016-0306-8},

year = {2017},

date = {2017-01-01},

journal = {Space Science Reviews},

volume = {210},

pages = {183-226},

abstract = {The empirical investigation of the magnetic field in the outer solar

atmosphere is a very important challenge in astrophysics. To this end,

we need to identify, measure and interpret observable quantities

sensitive to the magnetism of the upper chromosphere, transition region

and corona. This paper provides an overview of the physics and

diagnostic potential of spectropolarimetry in permitted spectral lines

of the ultraviolet solar spectrum, such as the Mg ii h and k lines

around 2800 AA, the hydrogen Lyman-$alpha$ line at 1216 AA, and the Lyman-$alpha$ line of He ii at 304 AA. The outer solar

atmosphere is an optically pumped vapor and the linear polarization of

such spectral lines is dominated by the atomic level polarization

produced by the absorption and scattering of anisotropic radiation. Its

modification by the action of the Hanle and Zeeman effects in the

inhomogeneous and dynamic solar atmosphere needs to be carefully

understood because it encodes the magnetic field information. The

circular polarization induced by the Zeeman effect in some ultraviolet

lines (e.g., Mg ii h amp k) is also of diagnostic interest, especially

for probing the outer solar atmosphere in plages and more active

regions. The few (pioneering) observational attempts carried out so far

to measure the ultraviolet spectral line polarization produced by

optically pumped atoms in the upper chromosphere, transition region and

corona are also discussed. We emphasize that ultraviolet

spectropolarimetry is a key gateway to the outer atmosphere of the Sun

and of other stars. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

atmosphere is a very important challenge in astrophysics. To this end,

we need to identify, measure and interpret observable quantities

sensitive to the magnetism of the upper chromosphere, transition region

and corona. This paper provides an overview of the physics and

diagnostic potential of spectropolarimetry in permitted spectral lines

of the ultraviolet solar spectrum, such as the Mg ii h and k lines

around 2800 AA, the hydrogen Lyman-$alpha$ line at 1216 AA, and the Lyman-$alpha$ line of He ii at 304 AA. The outer solar

atmosphere is an optically pumped vapor and the linear polarization of

such spectral lines is dominated by the atomic level polarization

produced by the absorption and scattering of anisotropic radiation. Its

modification by the action of the Hanle and Zeeman effects in the

inhomogeneous and dynamic solar atmosphere needs to be carefully

understood because it encodes the magnetic field information. The

circular polarization induced by the Zeeman effect in some ultraviolet

lines (e.g., Mg ii h amp k) is also of diagnostic interest, especially

for probing the outer solar atmosphere in plages and more active

regions. The few (pioneering) observational attempts carried out so far

to measure the ultraviolet spectral line polarization produced by

optically pumped atoms in the upper chromosphere, transition region and

corona are also discussed. We emphasize that ultraviolet

spectropolarimetry is a key gateway to the outer atmosphere of the Sun

and of other stars.

Ishikawa, R; Bueno, J Trujillo; Uitenbroek, H; Kubo, M; Tsuneta, S; Goto, M; Kano, R; Narukage, N; Bando, T; Katsukawa, Y; Ishikawa, S; Giono, G; Suematsu, Y; Hara, H; Shimizu, T; Sakao, T; Winebarger, A; Kobayashi, K; Cirtain, J; Champey, P; Auch`ere, F; v Stv epán, J; Belluzzi, L; Ramos, A Asensio; Sainz, R Manso; Pontieu, B De; Ichimoto, K; Carlsson, M; Casini, R

Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines Journal Article

In: Astrophysical Journal, vol. 841, pp. 31, 2017.

@article{2017ApJ...841...31I,

title = {Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines},

author = {R {Ishikawa} and J {Trujillo Bueno} and H {Uitenbroek} and M {Kubo} and S {Tsuneta} and M {Goto} and R {Kano} and N {Narukage} and T {Bando} and Y {Katsukawa} and S {Ishikawa} and G {Giono} and Y {Suematsu} and H {Hara} and T {Shimizu} and T {Sakao} and A {Winebarger} and K {Kobayashi} and J {Cirtain} and P {Champey} and F {Auch{`e}re} and J {{v S}t{v e}pán} and L {Belluzzi} and A {Asensio Ramos} and R {Manso Sainz} and B {De Pontieu} and K {Ichimoto} and M {Carlsson} and R {Casini}},

url = {http://adsabs.harvard.edu/abs/2017ApJ...841...31I},

doi = {10.3847/1538-4357/aa6ca9},

year = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {Astrophysical Journal},

volume = {841},

pages = {31},

abstract = {The Chromospheric Lyman-α Spectro-Polarimeter is a sounding rocket

experiment that has provided the first successful measurement of the

linear polarization produced by scattering processes in the hydrogen

Lyα line (121.57 nm) radiation of the solar disk. In this paper,

we report that the Si III line at 120.65 nm also shows scattering

polarization and we compare the scattering polarization signals observed

in the Lyα and Si III lines in order to search for observational

signatures of the Hanle effect. We focus on four selected bright

structures and investigate how the U/I spatial variations vary between

the Lyα wing, the Lyα core, and the Si III line as a

function of the total unsigned photospheric magnetic flux estimated from

Solar Dynamics Observatory/Helioseismic and Magnetic Imager

observations. In an internetwork region, the Lyα core shows an

antisymmetric spatial variation across the selected bright structure,

but it does not show it in other more magnetized regions. In the Si III

line, the spatial variation of U/I deviates from the above-mentioned

antisymmetric shape as the total unsigned photospheric magnetic flux

increases. A plausible explanation of this difference is the operation

of the Hanle effect. We argue that diagnostic techniques based on the

scattering polarization observed simultaneously in two spectral lines

with very different sensitivities to the Hanle effect, like Lyα

and Si III, are of great potential interest for exploring the magnetism

of the upper solar chromosphere and transition region. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

experiment that has provided the first successful measurement of the

linear polarization produced by scattering processes in the hydrogen

Lyα line (121.57 nm) radiation of the solar disk. In this paper,

we report that the Si III line at 120.65 nm also shows scattering

polarization and we compare the scattering polarization signals observed

in the Lyα and Si III lines in order to search for observational

signatures of the Hanle effect. We focus on four selected bright

structures and investigate how the U/I spatial variations vary between

the Lyα wing, the Lyα core, and the Si III line as a

function of the total unsigned photospheric magnetic flux estimated from

Solar Dynamics Observatory/Helioseismic and Magnetic Imager

observations. In an internetwork region, the Lyα core shows an

antisymmetric spatial variation across the selected bright structure,

but it does not show it in other more magnetized regions. In the Si III

line, the spatial variation of U/I deviates from the above-mentioned

antisymmetric shape as the total unsigned photospheric magnetic flux

increases. A plausible explanation of this difference is the operation

of the Hanle effect. We argue that diagnostic techniques based on the

scattering polarization observed simultaneously in two spectral lines

with very different sensitivities to the Hanle effect, like Lyα

and Si III, are of great potential interest for exploring the magnetism

of the upper solar chromosphere and transition region.

### 2016

Kubo, M.; Katsukawa, Y.; Suematsu, Y.; Kano, R.; Bando, T.; Narukage, N.; Ishikawa, R.; Hara, H.; Giono, G.; Tsuneta, S.; Ishikawa, S.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.; Auch`ere, F.; Trujillo Bueno, J.; Asensio Ramos, A.; v Stv ep'an, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M.

Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman α Spectropolarimeter (CLASP) Journal Article

In: Astrophysical Journal, vol. 832, pp. 141, 2016.

@article{2016ApJ...832..141K,

title = {Discovery of Ubiquitous Fast-Propagating Intensity Disturbances by the Chromospheric Lyman α Spectropolarimeter (CLASP)},

author = {Kubo, M. and Katsukawa, Y. and Suematsu, Y. and Kano, R. and

Bando, T. and Narukage, N. and Ishikawa, R. and Hara, H. and

Giono, G. and Tsuneta, S. and Ishikawa, S. and Shimizu, T. and

Sakao, T. and Winebarger, A. and Kobayashi, K. and Cirtain, J. and

Champey, P. and Auch`ere, F. and Trujillo Bueno, J. and

Asensio Ramos, A. and v Stv ep'an, J. and Belluzzi, L. and

Manso Sainz, R. and De Pontieu, B. and Ichimoto, K. and

Carlsson, M. and Casini, R. and Goto, M.},

url = {http://adsabs.harvard.edu/abs/2016ApJ...832..141K},

doi = {10.3847/0004-637X/832/2/141},

year = {2016},

date = {2016-12-01},

journal = {Astrophysical Journal},

volume = {832},

pages = {141},

abstract = {High-cadence observations by the slit-jaw (SJ) optics system of the sounding rocket experiment known as the Chromospheric Lyman Alpha Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances that recurrently propagate in either the chromosphere or the transition region or both at a speed much higher than the speed of sound. The CLASP/SJ instrument provides a time series of two-dimensional images taken with broadband filters centered on the Lyα line at a 0.6 s cadence. The multiple fast-propagating intensity disturbances appear in the quiet Sun and in an active region, and they are clearly detected in at least 20 areas in a field of view of 527″ × 527″ during the 5 minute observing time. The apparent speeds of the intensity disturbances range from 150 to 350 km s-1, and they are comparable to the local Alfvén speed in the transition region. The intensity disturbances tend to propagate along bright elongated structures away from areas with strong photospheric magnetic fields. This suggests that the observed fast-propagating intensity disturbances are related to the magnetic canopy structures. The maximum distance traveled by the intensity disturbances is about 10″, and the widths are a few arcseconds, which are almost determined by a pixel size of 1.″03. The timescale of each intensity pulse is shorter than 30 s. One possible explanation for the fast-propagating intensity disturbances observed by CLASP is magnetohydrodynamic fast-mode waves. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Cortesi, S.; Cagnotti, M.; Bianda, M.; Ramelli, R.; Manna, A.

Sunspot Observations and Counting at Specola Solare Ticinese in Locarno Since 1957 Journal Article

In: Solar Physics, vol. 291, pp. 3075-3080, 2016.

@article{2016SoPh..291.3075C,

title = {Sunspot Observations and Counting at Specola Solare Ticinese in Locarno Since 1957},

author = {Cortesi, S. and Cagnotti, M. and Bianda, M. and Ramelli, R. and Manna, A.},

url = {https://arxiv.org/pdf/1602.07998v1

http://adsabs.harvard.edu/abs/2016SoPh..291.3075C},

doi = {10.1007/s11207-016-0872-7},

year = {2016},

date = {2016-11-01},

urldate = {2016-11-01},

journal = {Solar Physics},

volume = {291},

pages = {3075-3080},

abstract = {Specola Solare Ticinese is an observatory dedicated to sunspot number counting. It was constructed in 1957 in Locarno, Southern Switzerland, as an external observing station of the Zurich Federal Observatory. When the responsibility of the determination of the International Sunspot Number was assumed by the Royal Observatory of Belgium in 1981, Specola Solare Ticinese was given the role of pilot station, with the aim of preserving the continuity in the counting method. We report the observing procedure and counting rules applied in Locarno.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.

The Magnetic Sensitivity of the Mg ii k Line to the Joint Action of Hanle, Zeeman, and Magneto-optical Effects Journal Article

In: The Astrophysical Journal Letters, vol. 831, pp. L15, 2016.

@article{2016ApJ...831L..15A,

title = {The Magnetic Sensitivity of the Mg ii k Line to the Joint Action of Hanle, Zeeman, and Magneto-optical Effects},

author = {Alsina Ballester, E. and Belluzzi, L. and Trujillo Bueno, J.},

url = {https://arxiv.org/abs/1610.00649},

doi = {10.3847/2041-8205/831/2/L15},

year = {2016},

date = {2016-11-01},

journal = {The Astrophysical Journal Letters},

volume = {831},

pages = {L15},

abstract = {We highlight the main results of a radiative transfer investigation on the magnetic sensitivity of the solar Mg ii k resonance line at 2795.5 Å, accounting for the joint action of the Hanle and Zeeman effects as well as partial frequency redistribution phenomena. We confirm that at the line center, the linear polarization signals produced by scattering processes are measurable, and that they are sensitive, via the Hanle effect, to magnetic fields with strengths between 5 and 50 G, approximately. We also show that the Zeeman effect produces conspicuous circular polarization signals, especially for longitudinal fields stronger than 50 G, which can be used to estimate the magnetization of the solar chromosphere via the familiar magnetograph formula. The most novel result is that magneto-optical effects produce, in the wings of the line, a decrease of the Q/I scattering polarization pattern and the appearance of U/I signals (i.e., a rotation of the plane of linear polarization). This sensitivity of the Q/I and U/I wing signals to both weak (∼5 G) and stronger magnetic fields expands the scientific interest of the Mg ii k line for probing the chromosphere in quiet and active regions of the Sun. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Kato, Y.; Steiner, O.; Hansteen, V.; Gudiksen, B.; Wedemeyer, S.; Carlsson, M.

Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath Journal Article

In: Astrophysical Journal, vol. 827, pp. 7, 2016.

@article{2016ApJ...827....7K,

title = {Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath},

author = {Kato, Y. and Steiner, O. and Hansteen, V. and Gudiksen, B. and

Wedemeyer, S. and Carlsson, M.},

url = {https://arxiv.org/abs/1606.08826},

doi = {10.3847/0004-637X/827/1/7},

year = {2016},

date = {2016-11-01},

journal = {Astrophysical Journal},

volume = {827},

pages = {7},

abstract = {Using radiation magnetohydrodynamic simulations of the solar atmospheric

layers from the upper convection zone to the lower corona, we

investigate the self-consistent excitation of slow magneto-acoustic body

waves (slow modes) in a magnetic flux concentration. We find that the

convective downdrafts in the close surroundings of a two-dimensional

flux slab łdquopumprdquo the plasma inside it in the downward

direction. This action produces a downflow inside the flux slab, which

encompasses ever higher layers, causing an upwardly propagating

rarefaction wave. The slow mode, excited by the adiabatic compression of

the downflow near the optical surface, travels along the magnetic field

in the upward direction at the tube speed. It develops into a shock wave

at chromospheric heights, where it dissipates, lifts the transition

region, and produces an offspring in the form of a compressive wave that

propagates further into the corona. In the wake of downflows and

propagating shock waves, the atmosphere inside the flux slab in the

chromosphere and higher tends to oscillate with a period of $nu$ ap 4

mHz. We conclude that this process of łdquomagnetic pumpingrdquo is

a most plausible mechanism for the direct generation of longitudinal

chromospheric and coronal compressive waves within magnetic flux

concentrations, and it may provide an important heat source in the

chromosphere. It may also be responsible for certain types of dynamic

fibrils. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

layers from the upper convection zone to the lower corona, we

investigate the self-consistent excitation of slow magneto-acoustic body

waves (slow modes) in a magnetic flux concentration. We find that the

convective downdrafts in the close surroundings of a two-dimensional

flux slab łdquopumprdquo the plasma inside it in the downward

direction. This action produces a downflow inside the flux slab, which

encompasses ever higher layers, causing an upwardly propagating

rarefaction wave. The slow mode, excited by the adiabatic compression of

the downflow near the optical surface, travels along the magnetic field

in the upward direction at the tube speed. It develops into a shock wave

at chromospheric heights, where it dissipates, lifts the transition

region, and produces an offspring in the form of a compressive wave that

propagates further into the corona. In the wake of downflows and

propagating shock waves, the atmosphere inside the flux slab in the

chromosphere and higher tends to oscillate with a period of $nu$ ap 4

mHz. We conclude that this process of łdquomagnetic pumpingrdquo is

a most plausible mechanism for the direct generation of longitudinal

chromospheric and coronal compressive waves within magnetic flux

concentrations, and it may provide an important heat source in the

chromosphere. It may also be responsible for certain types of dynamic

fibrils.

Stenflo, J. O.

Transition of the Sunspot Number from Zurich to Brussels in 1980: A Personal Perspective Journal Article

In: Solar Physics, vol. 291, pp. 2487-2492, 2016.

@article{2016SoPh..291.2487S,

title = {Transition of the Sunspot Number from Zurich to Brussels in 1980: A Personal Perspective},

author = {Stenflo, J.~O.},

url = {https://arxiv.org/abs/1512.06229},

doi = {10.1007/s11207-015-0837-2},

year = {2016},

date = {2016-11-01},

journal = {Solar Physics},

volume = {291},

pages = {2487-2492},

abstract = {The Swiss Federal Observatory, which had been founded in 1863 by Rudolf

Wolf, was dissolved in connection with the retirement of Max Waldmeier

in 1979. The determination of the Zurich sunpot number, which had been a

cornerstone activity of the observatory, was then discontinued by ETH

Zurich. A smooth transition of the responsibility for the sunspot number

from Zurich to Brussels was achieved in 1980, however, through which it

was possible to avoid a discontinuity in this important time series.

Here we describe the circumstances that led to the termination in

Zurich, how Brussels was chosen for the succession, and how the transfer

was accomplished. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Wolf, was dissolved in connection with the retirement of Max Waldmeier

in 1979. The determination of the Zurich sunpot number, which had been a

cornerstone activity of the observatory, was then discontinued by ETH

Zurich. A smooth transition of the responsibility for the sunspot number

from Zurich to Brussels was achieved in 1980, however, through which it

was possible to avoid a discontinuity in this important time series.

Here we describe the circumstances that led to the termination in

Zurich, how Brussels was chosen for the succession, and how the transfer

was accomplished.

Carlin, E. S.; Bianda, M.

The Key Role of Solar Dynamics in the Chromospheric Hanle Polarization Journal Article

In: The Astrophysical Journal Letters, vol. 831, no. 1, pp. L5, 2016.

@article{2041-8205-831-1-L5,

title = {The Key Role of Solar Dynamics in the Chromospheric Hanle Polarization},

author = {E. S. Carlin and M. Bianda},

url = {http://www.irsol.usi.ch/wp-content/uploads/2016/10/apjl_831_1_L5o-1.pdf},

year = {2016},

date = {2016-10-25},

journal = {The Astrophysical Journal Letters},

volume = {831},

number = {1},

pages = {L5},

abstract = {The quantum theory of polarized light allows one to model scattering in the solar atmosphere for inferring its properties. This powerful approach has revealed two key long-standing problems in solar physics: the puzzling dilemmas between theory and observations in several anomalously polarized spectral lines and the need for inferring the ubiquitous weak chromospheric magnetic fields, which requires discriminating the Hanle effect in dynamic optically thick plasmas. However, the ever-present dynamics, i.e., the temporal evolution of heatings and macroscopic motions, has been widely disregarded when modeling and interpreting the scattering polarization. This has hindered a consistent theoretical solution to the puzzle while falsifying the Hanle diagnosis. Here, we show that the dynamical evolution is a keystone for solving both problems because its systematic impact allows an explanation of the observations from “anomalous” instantaneous polarization signals. Evolution accounted for, we reproduce amplitudes and (spectral and spatial) shapes of the Ca i 4227 Å polarization at solar disk center, identifying a restrictive arrangement of magnetic fields, kinematics, heatings, and spatio-temporal resolution. We find that the joint action of dynamics, Hanle effect, and low temporal resolutions mimics Zeeman linear polarization profiles, the true weak-field Zeeman signals being negligible. Our results allow reinterpretation of many polarization signals of the solar spectra and support time-dependent scattering polarization as a powerful tool for deciphering the spatio-temporal distribution of chromospheric heatings and fields. This approach may be a key aid in developing the Hanle diagnosis for the solar atmosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Privitera, G.; Meynet, G.; Eggenberger, P.; Vidotto, A. A.; Villaver, E.; Bianda, M.

Star-planet interactions. II. Is planet engulfment the origin of fast rotating red giants? Journal Article

In: Astronomy and Astrophysics, vol. 593, pp. A128, 2016.

@article{2016A&A...593A.128P,

title = {Star-planet interactions. II. Is planet engulfment the origin of fast rotating red giants?},

author = {Privitera, G. and Meynet, G. and Eggenberger, P. and Vidotto, A.A. and

Villaver, E. and Bianda, M.},

url = {http://www.aanda.org/articles/aa/abs/2016/09/aa28758-16/aa28758-16.html},

doi = {10.1051/0004-6361/201628758},

year = {2016},

date = {2016-09-01},

journal = {Astronomy and Astrophysics},

volume = {593},

pages = {A128},

abstract = {Context. Fast rotating red giants in the upper part of the red giant

branch have surface velocities that cannot be explained by single star

evolution.

Aims: We check whether tides between a star and a

planet followed by planet engulfment can indeed accelerate the surface

rotation of red giants for a sufficiently long time to produce these

fast rotating red giants.

Methods: We studied how the surface

rotation velocity at the stellar surface evolves using rotating stellar

models, accounting for the redistribution of the angular momentum inside

the star by different transport mechanisms, the exchanges of angular

momentum between the planet orbit and the star before the engulfment,

and for the deposition of angular momentum inside the star at the

engulfment. We considered different situations with masses of stars in

the range between 1.5 and 2.5 M$_⊙$, masses of the planets

between 1 and 15 M$_J$ (Jupiter mass), and initial semimajor

axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is

0.02.

Results: We show that the surface velocities reached at the

end of the orbital decay due to tidal forces and planet engulfment can

be similar to values observed for fast rotating red giants. This surface

velocity then decreases when the star evolves along the red giant branch

but at a sufficiently slow pace to allowing stars to be detected with

such a high velocity. More quantitatively, star-planet interaction can

produce a rapid acceleration of the surface of the star, above values

equal to 8 km s$^-1$, for periods lasting up to more than 30%

the red giant branch phase. As found already by previous works, the

changes of the surface carbon isotopic ratios produced by the dilution

of the planetary material into the convective envelope is modest. The

increase of the lithium abundance due to this effect might be much more

important, however lithium may be affected by many different, still

uncertain, processes. Thus any lithium measurement can hardly be taken

as a support or argument against any star-planet interaction.

Conclusions: The acceleration of the stellar surface to rotation

velocities above limits that depend on the surface gravity does appear

at the moment to be the clearest signature of a star-planet interaction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

branch have surface velocities that cannot be explained by single star

evolution. <BR /> Aims: We check whether tides between a star and a

planet followed by planet engulfment can indeed accelerate the surface

rotation of red giants for a sufficiently long time to produce these

fast rotating red giants. <BR /> Methods: We studied how the surface

rotation velocity at the stellar surface evolves using rotating stellar

models, accounting for the redistribution of the angular momentum inside

the star by different transport mechanisms, the exchanges of angular

momentum between the planet orbit and the star before the engulfment,

and for the deposition of angular momentum inside the star at the

engulfment. We considered different situations with masses of stars in

the range between 1.5 and 2.5 M$_⊙$, masses of the planets

between 1 and 15 M$_J$ (Jupiter mass), and initial semimajor

axis between 0.5 and 1.5 au. The metallicity Z for our stellar models is

0.02. <BR /> Results: We show that the surface velocities reached at the

end of the orbital decay due to tidal forces and planet engulfment can

be similar to values observed for fast rotating red giants. This surface

velocity then decreases when the star evolves along the red giant branch

but at a sufficiently slow pace to allowing stars to be detected with

such a high velocity. More quantitatively, star-planet interaction can

produce a rapid acceleration of the surface of the star, above values

equal to 8 km s$^-1$, for periods lasting up to more than 30%

the red giant branch phase. As found already by previous works, the

changes of the surface carbon isotopic ratios produced by the dilution

of the planetary material into the convective envelope is modest. The

increase of the lithium abundance due to this effect might be much more

important, however lithium may be affected by many different, still

uncertain, processes. Thus any lithium measurement can hardly be taken

as a support or argument against any star-planet interaction. <BR />

Conclusions: The acceleration of the stellar surface to rotation

velocities above limits that depend on the surface gravity does appear

at the moment to be the clearest signature of a star-planet interaction.

Privitera, G.; Meynet, G.; Eggenberger, P.; Georgy, C.; Ekström, S.; Vidotto, A. A.; Bianda, M.; Villaver, E.; ud-Doula, A.

High surface magnetic field in red giants as a new signature of planet engulfment? Journal Article

In: Astronomy and Astrophysics, vol. 593, pp. L15, 2016.

@article{2016A&A...593L..15P,

title = {High surface magnetic field in red giants as a new signature of planet engulfment?},

author = {Privitera, G. and Meynet, G. and Eggenberger, P. and Georgy, C. and

Ekström, S. and Vidotto, A.A. and Bianda, M. and

Villaver, E. and ud-Doula, A.},

url = {http://www.aanda.org/articles/aa/full_html/2016/09/aa29142-16/aa29142-16.html},

doi = {10.1051/0004-6361/201629142},

year = {2016},

date = {2016-09-01},

journal = {Astronomy and Astrophysics},

volume = {593},

pages = {L15},

abstract = {Context. Red giant stars may engulf planets. This may increase the

rotation rate of their convective envelope, which could lead to strong

dynamo-triggered magnetic fields.

Aims: We explore the

possibility of generating magnetic fields in red giants that have gone

through the process of a planet engulfment. We compare them with similar

models that evolve without any planets. We discuss the impact of

magnetic braking through stellar wind on the evolution of the surface

velocity of the parent star.

Methods: By studying rotating

stellar models with and without planets and an empirical relation

between the Rossby number and the surface magnetic field, we deduced the

evolution of the surface magnetic field along the red giant branch. The

effects of stellar wind magnetic braking were explored using a relation

deduced from magnetohydrodynamics simulations.

Results: The

stellar evolution model of a red giant with 1.7 M$_⊙$

without planet engulfment and with a time-averaged rotation velocity

during the main sequence equal to 100 km s$^-1$ shows a surface

magnetic field triggered by convection that is stronger than 10 G only

at the base of the red giant branch, that is, for gravities log g$gt$ 3.

When a planet engulfment occurs, this magnetic field can also appear at

much lower gravities, that is, at much higher luminosities along the red

giant branch. The engulfment of a 15 M$_J$ planet typically

produces a dynamo-triggered magnetic field stronger than 10 G for

gravities between 2.5 and 1.9. We show that for reasonable magnetic

braking laws for the wind, the high surface velocity reached after a

planet engulfment may be maintained sufficiently long to be observable.

Conclusions: High surface magnetic fields for red giants in the

upper part of the red giant branch are a strong indication of a planet

engulfment or of an interaction with a companion. Our theory can be

tested by observing fast-rotating red giants such as HD 31994, Tyc

0347-00762-1, Tyc 5904-00513-1, and Tyc 6054-01204-1 and by determining

whether they show magnetic fields.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

rotation rate of their convective envelope, which could lead to strong

dynamo-triggered magnetic fields. <BR /> Aims: We explore the

possibility of generating magnetic fields in red giants that have gone

through the process of a planet engulfment. We compare them with similar

models that evolve without any planets. We discuss the impact of

magnetic braking through stellar wind on the evolution of the surface

velocity of the parent star. <BR /> Methods: By studying rotating

stellar models with and without planets and an empirical relation

between the Rossby number and the surface magnetic field, we deduced the

evolution of the surface magnetic field along the red giant branch. The

effects of stellar wind magnetic braking were explored using a relation

deduced from magnetohydrodynamics simulations. <BR /> Results: The

stellar evolution model of a red giant with 1.7 M$_⊙$

without planet engulfment and with a time-averaged rotation velocity

during the main sequence equal to 100 km s$^-1$ shows a surface

magnetic field triggered by convection that is stronger than 10 G only

at the base of the red giant branch, that is, for gravities log g$gt$ 3.

When a planet engulfment occurs, this magnetic field can also appear at

much lower gravities, that is, at much higher luminosities along the red

giant branch. The engulfment of a 15 M$_J$ planet typically

produces a dynamo-triggered magnetic field stronger than 10 G for

gravities between 2.5 and 1.9. We show that for reasonable magnetic

braking laws for the wind, the high surface velocity reached after a

planet engulfment may be maintained sufficiently long to be observable.

<BR /> Conclusions: High surface magnetic fields for red giants in the

upper part of the red giant branch are a strong indication of a planet

engulfment or of an interaction with a companion. Our theory can be

tested by observing fast-rotating red giants such as HD 31994, Tyc

0347-00762-1, Tyc 5904-00513-1, and Tyc 6054-01204-1 and by determining

whether they show magnetic fields.

Calvo, F.; Steiner, O.; Freytag, B.

Non-magnetic photospheric bright points in 3D simulations of the solar atmosphere Journal Article

In: Astronomy and Astrophysics, vol. 596, pp. A43, 2016.

@article{2016A&A...596A..43C,

title = {Non-magnetic photospheric bright points in 3D simulations of the solar atmosphere},

author = {Calvo, F. and Steiner, O. and Freytag, B.},

url = {http://www.aanda.org/articles/aa/abs/2016/12/aa28649-16/aa28649-16.html

https://arxiv.org/abs/1612.04278},

doi = {10.1051/0004-6361/201628649},

year = {2016},

date = {2016-08-01},

journal = {Astronomy and Astrophysics},

volume = {596},

pages = {A43},

abstract = {Context. Small-scale bright features in the photosphere of the Sun, such

as faculae or G-band bright points, appear in connection with

small-scale magnetic flux concentrations.

Aims: Here we report on

a new class of photospheric bright points that are free of magnetic

fields. So far, these are visible in numerical simulations only. We

explore conditions required for their observational detection.

Methods: Numerical radiation (magneto-)hydrodynamic simulations of the

near-surface layers of the Sun were carried out. The magnetic field-free

simulations show tiny bright points, reminiscent of magnetic bright

points, only smaller. A simple toy model for these non-magnetic bright

points (nMBPs) was established that serves as a base for the development

of an algorithm for their automatic detection. Basic physical properties

of 357 detected nMBPs were extracted and statistically evaluated. We

produced synthetic intensity maps that mimic observations with various

solar telescopes to obtain hints on their detectability.

Results:

The nMBPs of the simulations show a mean bolometric intensity contrast

with respect to their intergranular surroundings of approximately 20%, a

size of 60-80 km, and the isosurface of optical depth unity is at their

location depressed by 80-100 km. They are caused by swirling downdrafts

that provide, by means of the centripetal force, the necessary pressure

gradient for the formation of a funnel of reduced mass density that

reaches from the subsurface layers into the photosphere. Similar,

frequently occurring funnels that do not reach into the photosphere, do

not produce bright points.

Conclusions: Non-magnetic bright

points are the observable manifestation of vertically extending vortices

(vortex tubes) in the photosphere. The resolving power of 4-m-class

telescopes, such as the DKIST, is needed for an unambiguous detection of

them.

The movie associated to Fig. 1 is available at http://www.aanda.org },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

as faculae or G-band bright points, appear in connection with

small-scale magnetic flux concentrations. <BR /> Aims: Here we report on

a new class of photospheric bright points that are free of magnetic

fields. So far, these are visible in numerical simulations only. We

explore conditions required for their observational detection. <BR />

Methods: Numerical radiation (magneto-)hydrodynamic simulations of the

near-surface layers of the Sun were carried out. The magnetic field-free

simulations show tiny bright points, reminiscent of magnetic bright

points, only smaller. A simple toy model for these non-magnetic bright

points (nMBPs) was established that serves as a base for the development

of an algorithm for their automatic detection. Basic physical properties

of 357 detected nMBPs were extracted and statistically evaluated. We

produced synthetic intensity maps that mimic observations with various

solar telescopes to obtain hints on their detectability. <BR /> Results:

The nMBPs of the simulations show a mean bolometric intensity contrast

with respect to their intergranular surroundings of approximately 20%, a

size of 60-80 km, and the isosurface of optical depth unity is at their

location depressed by 80-100 km. They are caused by swirling downdrafts

that provide, by means of the centripetal force, the necessary pressure

gradient for the formation of a funnel of reduced mass density that

reaches from the subsurface layers into the photosphere. Similar,

frequently occurring funnels that do not reach into the photosphere, do

not produce bright points. <BR /> Conclusions: Non-magnetic bright

points are the observable manifestation of vertically extending vortices

(vortex tubes) in the photosphere. The resolving power of 4-m-class

telescopes, such as the DKIST, is needed for an unambiguous detection of

them.

The movie associated to Fig. 1 is available at <A href=``http://www.aanda.org/10.1051/0004-6361/201628649/olm''>http://www.aanda.org</A>

Privitera, G.; Meynet, G.; Eggenberger, P.; Vidotto, A. A.; Villaver, E.; Bianda, M.

Star-planet interactions. I. Stellar rotation and planetary orbits Journal Article

In: Astronomy and Astrophysics, vol. 591, pp. A45, 2016.

@article{2016A&A...591A..45P,

title = {Star-planet interactions. I. Stellar rotation and planetary orbits},

author = {Privitera, G. and Meynet, G. and Eggenberger, P. and Vidotto, A.A. and

Villaver, E. and Bianda, M.},

url = {http://www.irsol.usi.ch/wp-content/uploads/2016/10/aa28044-15.pdf},

doi = {10.1051/0004-6361/201528044},

year = {2016},

date = {2016-07-01},

journal = {Astronomy and Astrophysics},

volume = {591},

pages = {A45},

abstract = {Context. As a star evolves, planet orbits change over time owing to

tidal interactions, stellar mass losses, friction and gravitational drag

forces, mass accretion, and evaporation on/by the planet. Stellar

rotation modifies the structure of the star and therefore the way these

different processes occur. Changes in orbits, subsequently, have an

impact on the rotation of the star.

Aims: Models that account in

a consistent way for these interactions between the orbital evolution of

the planet and the evolution of the rotation of the star are still

missing. The present work is a first attempt to fill this gap.

Methods: We compute the evolution of stellar models including a

comprehensive treatment of rotational effects, together with the

evolution of planetary orbits, so that the exchanges of angular momentum

between the star and the planetary orbit are treated in a

self-consistent way. The evolution of the rotation of the star accounts

for the angular momentum exchange with the planet and also follows the

effects of the internal transport of angular momentum and chemicals.

These rotating models are computed for initial masses of the host star

between 1.5 and 2.5 M$_⊙$, with initial surface angular

velocities equal to 10 and 50% of the critical velocity on the zero age main sequence (ZAMS), for a metallicity Z = 0.02, with and without tidal

interactions with a planet. We consider planets with masses between 1

and 15 Jupiter masses (M$_J$), which are beginning their

evolution at various distances between 0.35 and 4.5 au.

Results:

We demonstrate that rotating stellar models without tidal interactions

and without any wind magnetic braking during the red giant phase can

well reproduce the surface rotations of the bulk of red giants. However,

models without any interactions cannot account for fast rotating red

giants in the upper part of the red giant branch, where these models,

whatever the initial rotation considered on the ZAMS, always predict

very low velocities. For these stars, some interaction with a companion

is highly probable and the present rotating stellar models with planets

confirm that tidal interaction can reproduce their high surface

velocities. We also show that the minimum distance between the planet

and the star on the ZAMS, which enables the planet to avoid engulfment

and survive (i.e. the survival limit) is decreased around faster

rotating stars.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

tidal interactions, stellar mass losses, friction and gravitational drag

forces, mass accretion, and evaporation on/by the planet. Stellar

rotation modifies the structure of the star and therefore the way these

different processes occur. Changes in orbits, subsequently, have an

impact on the rotation of the star. <BR /> Aims: Models that account in

a consistent way for these interactions between the orbital evolution of

the planet and the evolution of the rotation of the star are still

missing. The present work is a first attempt to fill this gap. <BR />

Methods: We compute the evolution of stellar models including a

comprehensive treatment of rotational effects, together with the

evolution of planetary orbits, so that the exchanges of angular momentum

between the star and the planetary orbit are treated in a

self-consistent way. The evolution of the rotation of the star accounts

for the angular momentum exchange with the planet and also follows the

effects of the internal transport of angular momentum and chemicals.

These rotating models are computed for initial masses of the host star

between 1.5 and 2.5 M$_⊙$, with initial surface angular

velocities equal to 10 and 50% of the critical velocity on the zero age main sequence (ZAMS), for a metallicity Z = 0.02, with and without tidal

interactions with a planet. We consider planets with masses between 1

and 15 Jupiter masses (M$_J$), which are beginning their

evolution at various distances between 0.35 and 4.5 au. <BR /> Results:

We demonstrate that rotating stellar models without tidal interactions

and without any wind magnetic braking during the red giant phase can

well reproduce the surface rotations of the bulk of red giants. However,

models without any interactions cannot account for fast rotating red

giants in the upper part of the red giant branch, where these models,

whatever the initial rotation considered on the ZAMS, always predict

very low velocities. For these stars, some interaction with a companion

is highly probable and the present rotating stellar models with planets

confirm that tidal interaction can reproduce their high surface

velocities. We also show that the minimum distance between the planet

and the star on the ZAMS, which enables the planet to avoid engulfment

and survive (i.e. the survival limit) is decreased around faster

rotating stars.

Steiner, O.; Züger, F.; Belluzzi, L.

Polarized radiative transfer in discontinuous media Journal Article

In: Astronomy and Astrophysics, vol. 586, pp. A42, 2016.

@article{2016A&A...586A..42S,

title = {Polarized radiative transfer in discontinuous media},

author = {Steiner, O. and Züger, F. and Belluzzi, L.},

url = {http://www.aanda.org/articles/aa/abs/2016/02/aa27158-15/aa27158-15.html},

doi = {10.1051/0004-6361/201527158},

year = {2016},

date = {2016-01-01},

journal = {Astronomy and Astrophysics},

volume = {586},

pages = {A42},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Supriya, H. D.; Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.; Ravindra, B.

Polarized Line Formation with Lower-level Polarization and Partial Frequency Redistribution Journal Article

In: Astrophysical Journal, vol. 828, pp. 84, 2016.

@article{2016ApJ...828...84S,

title = {Polarized Line Formation with Lower-level Polarization and Partial Frequency Redistribution},

author = {Supriya, H.D. and Sampoorna, M. and Nagendra, K.N. and

Stenflo, J.~O. and Ravindra, B.},

doi = {10.3847/0004-637X/828/2/84},

year = {2016},

date = {2016-01-01},

journal = {Astrophysical Journal},

volume = {828},

pages = {84},

abstract = {In the well-established theories of polarized line formation with

partial frequency redistribution (PRD) for a two-level and two-term

atom, it is generally assumed that the lower level of the scattering

transition is unpolarized. However, the existence of unexplained

spectral features in some lines of the Second Solar Spectrum points

toward a need to relax this assumption. There exists a density matrix

theory that accounts for the polarization of all the atomic levels, but

it is based on the flat-spectrum approximation (corresponding to

complete frequency redistribution). In the present paper we propose a

numerical algorithm to solve the problem of polarized line formation in

magnetized media, which includes both the effects of PRD and the lower

level polarization (LLP) for a two-level atom. First we derive a

collisionless redistribution matrix that includes the combined effects

of the PRD and the LLP. We then solve the relevant transfer equation

using a two-stage approach. For illustration purposes, we consider two case studies in the non-magnetic regime, namely, the J $_ a $ = 1, J $_ b $ = 0 and J $_ a $ = J $_ b $ = 1,

where J $_ a $ and J $_ b $ represent the total angular

momentum quantum numbers of the lower and upper states, respectively.

Our studies show that the effects of LLP are significant only in the

line core. This leads us to propose a simplified numerical approach to

solve the concerned radiative transfer problem. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

partial frequency redistribution (PRD) for a two-level and two-term

atom, it is generally assumed that the lower level of the scattering

transition is unpolarized. However, the existence of unexplained

spectral features in some lines of the Second Solar Spectrum points

toward a need to relax this assumption. There exists a density matrix

theory that accounts for the polarization of all the atomic levels, but

it is based on the flat-spectrum approximation (corresponding to

complete frequency redistribution). In the present paper we propose a

numerical algorithm to solve the problem of polarized line formation in

magnetized media, which includes both the effects of PRD and the lower

level polarization (LLP) for a two-level atom. First we derive a

collisionless redistribution matrix that includes the combined effects

of the PRD and the LLP. We then solve the relevant transfer equation

using a two-stage approach. For illustration purposes, we consider two case studies in the non-magnetic regime, namely, the J $_ a $ = 1, J $_ b $ = 0 and J $_ a $ = J $_ b $ = 1,

where J $_ a $ and J $_ b $ represent the total angular

momentum quantum numbers of the lower and upper states, respectively.

Our studies show that the effects of LLP are significant only in the

line core. This leads us to propose a simplified numerical approach to

solve the concerned radiative transfer problem.

### 2015

Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.; Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P.

On the Evolution of Magnetic White Dwarfs Journal Article

In: Astrophysical Journal, vol. 812, pp. 19, 2015.

@article{2015ApJ...812...19T,

title = {On the Evolution of Magnetic White Dwarfs},

author = {Tremblay, P.-E. and Fontaine, G. and Freytag, B. and Steiner, O. and Ludwig, H.-G. and Steffen, M. and Wedemeyer, S. and Brassard, P.},

url = {http://adsabs.harvard.edu/abs/2015ApJ...812...19T},

doi = {10.1088/0004-637X/812/1/19},

year = {2015},

date = {2015-10-01},

urldate = {2015-10-01},

journal = {Astrophysical Journal},

volume = {812},

pages = {19},

abstract = {We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff ≲ 10,000 K cool significantly slower than non-magnetic degenerates.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Stenflo, J. O.

Physics of Polarized Scattering at Multi-level Atomic Systems Journal Article

In: Astrophysical Journal, vol. 801, pp. 70, 2015.

@article{2015ApJ...801...70S,

title = {Physics of Polarized Scattering at Multi-level Atomic Systems},

author = {Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2015ApJ...801...70S},

doi = {10.1088/0004-637X/801/1/70},

year = {2015},

date = {2015-03-01},

urldate = {2015-03-01},

journal = {Astrophysical Journal},

volume = {801},

pages = {70},

abstract = {The symmetric peak observed in linear polarization in the core of the solar sodium D1 line at 5896 Å has remained enigmatic since its discovery nearly two decades ago. One reason is that the theory of polarized scattering has not been experimentally tested for multi-level atomic systems in the relevant parameter domains, although the theory is continually being used for the interpretation of astrophysical observations. A laboratory experiment that was set up a decade ago to find out whether the D1 enigma is a problem of solar physics or quantum physics revealed that the D1 system has a rich polarization structure in situations where standard scattering theory predicts zero polarization, even when optical pumping of the m state populations of the hyperfine-split ground state is accounted for. Here we show that the laboratory results can be modeled in great quantitative detail if the theory is extended to include the coherences in both the initial and final states of the scattering process. Radiative couplings between the allowed dipole transitions generate coherences in the initial state. Corresponding coherences in the final state are then demanded by a phase closure selection rule. The experimental results for the well understood D2 line are used to constrain the two free parameters of the experiment, collision rate and optical depth, to suppress the need for free parameters when fitting the D1 results.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Carlin, E. S.; Asensio Ramos, A.

Chromospheric Diagnosis with Ca II Lines: Forward Modeling in Forward Scattering. I Journal Article

In: Astrophysical Journal, vol. 801, pp. 16, 2015.

@article{2015ApJ...801...16C,

title = {Chromospheric Diagnosis with Ca II Lines: Forward Modeling in Forward Scattering. I},

author = {Carlin, E.S. and Asensio Ramos, A.},

url = {http://adsabs.harvard.edu/abs/2015ApJ...801...16C},

doi = {10.1088/0004-637X/801/1/16},

year = {2015},

date = {2015-03-01},

urldate = {2015-03-01},

journal = {Astrophysical Journal},

volume = {801},

pages = {16},

abstract = {This paper presents a synthetic tomography of the quiet solar chromosphere formed by spatial maps of scattering polarization. It has been calculated for the Ca II 8498, 8542, and 3934 Å lines by solving the non-LTE radiative transfer problem of the second kind in a three-dimensional atmosphere model obtained from realistic magneto-hydrodynamical simulations. Our investigation focuses on the linear polarization signals induced by kinematics, radiation field anisotropy, and the Hanle effect in forward-scattering geometry. Thus, instead of considering slit profiles at the limb as normally done in the study of the second solar spectrum, we synthesize and analyze spatial maps of polarization at the disk center. This allows us to understand the spatial signatures of dynamics and magnetic field in the linear polarization in order to discriminate them observationally. Our results suggest some ideas for chromospheric diagnosis that will be developed throughout a series of papers. In particular, Hanle polarity inversion lines and dynamic Hanle diagrams are two concepts introduced in the present work. We find that chromospheric dynamics and magnetic field topology create spatial polarization fingerprints that trace the dynamic situation of the plasma and the magnetic field. This allows us to reconstruct the magnetic field intensity in the middle chromosphere using Stokes V along grooves of null linear polarization. We finally address the problems of diagnosing Hanle saturation and kinematic amplification of scattering signals using Hanle diagrams.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Belluzzi, L.; Trujillo Bueno, J.; Landi Degl'Innocenti, E.

Radiative Transfer Modeling of the Enigmatic Scattering Polarization in the Solar Na I D1 Line Journal Article

In: Astrophysical Journal, vol. 814, pp. 116, 2015.

@article{2015ApJ...814..116B,

title = {Radiative Transfer Modeling of the Enigmatic Scattering Polarization in the Solar Na I D1 Line},

author = {Belluzzi, L. and Trujillo Bueno, J. and Landi Degl'Innocenti, E.},

url = {http://adsabs.harvard.edu/abs/2015ApJ...814..116B},

doi = {10.1088/0004-637X/814/2/116},

year = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Astrophysical Journal},

volume = {814},

pages = {116},

abstract = {The modeling of the peculiar scattering polarization signals observed in some diagnostically important solar resonance lines requires the consideration of the detailed spectral structure of the incident radiation field as well as the possibility of ground level polarization, along with the atom's hyperfine structure and quantum interference between hyperfine F-levels pertaining either to the same fine structure J-level, or to different J-levels of the same term. Here we present a theoretical and numerical approach suitable for solving this complex non-LTE radiative transfer problem. This approach is based on the density-matrix metalevel theory (where each level is viewed as a continuous distribution of sublevels) and on accurate formal solvers of the transfer equations and efficient iterative methods. We show an application to the D-lines of Na i, with emphasis on the enigmatic D1 line, pointing out the observable signatures of the various physical mechanisms considered. We demonstrate that the linear polarization observed in the core of the D1 line may be explained by the effect that one gets when the detailed spectral structure of the anisotropic radiation responsible for the optical pumping is taken into account. This physical ingredient is capable of introducing significant scattering polarization in the core of the Na i D1 line without the need for ground-level polarization.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.

Isotropic Inelastic Collisions in a Multiterm Atom with Hyperfine Structure Journal Article

In: Astrophysical Journal, vol. 812, pp. 73, 2015.

@article{2015ApJ...812...73B,

title = {Isotropic Inelastic Collisions in a Multiterm Atom with Hyperfine Structure},

author = {Belluzzi, L. and Landi Degl'Innocenti, E. and Trujillo Bueno, J.},

url = {http://adsabs.harvard.edu/abs/2015ApJ...812...73B},

doi = {10.1088/0004-637X/812/1/73},

year = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Astrophysical Journal},

volume = {812},

pages = {73},

abstract = {A correct modeling of the scattering polarization profiles observed in some spectral lines of diagnostic interest, the sodium doublet being one of the most important examples, requires taking hyperfine structure (HFS) and quantum interference between different J-levels into account. An atomic model suitable for taking these physical ingredients into account is the so-called multiterm atom with HFS. In this work, we introduce and study the transfer and relaxation rates due to isotropic inelastic collisions with electrons, which enter the statistical equilibrium equations (SEE) for the atomic density matrix of this atomic model. Under the hypothesis that the electron-atom interaction is described by a dipolar operator, we provide useful relations between the rates describing the transfer and relaxation of quantum interference between different levels (whose numerical values are in most cases unknown) and the usual rates for the atomic level populations, for which experimental data and/or approximate theoretical expressions are generally available. For the particular case of a two-term atom with HFS, we present an analytical solution of the SEE for the spherical statistical tensors of the upper term, including both radiative and collisional processes, and we derive the expression of the emission coefficient in the four Stokes parameters. Finally, an illustrative application to the Na i D1 and D2 lines is presented.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Stenflo, J. O.

History of Solar Magnetic Fields Since George Ellery Hale Journal Article

In: Space Science Reviews, 2015.

@article{2015SSRv..tmp...83S,

title = {History of Solar Magnetic Fields Since George Ellery Hale},

author = {Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2015SSRv..tmp...83S},

doi = {10.1007/s11214-015-0198-z},

year = {2015},

date = {2015-01-01},

journal = {Space Science Reviews},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Stenflo, J. O.

FTS atlas of the Sun's spectrally resolved center-to-limb variation Journal Article

In: Astronomy and Astrophysics, vol. 573, pp. A74, 2015.

@article{2015A&A...573A..74S,

title = {FTS atlas of the Sun's spectrally resolved center-to-limb variation},

author = {Stenflo, J.O.},

url = {https://www.irsol.usi.ch/data/Papers/AA573_A74_2015.pdf},

doi = {10.1051/0004-6361/201424685},

year = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

journal = {Astronomy and Astrophysics},

volume = {573},

pages = {A74},

abstract = {The Sun's spectrum varies with center-to-limb distance, which is usually parameterized by μ = cosθ, where θ is the heliocentric angle. This variation is governed by the underlying temperature-density structure of the solar atmosphere. While the center-to-limb variation (CLV) of the continuous spectrum is well known and has been widely used for atmospheric modeling, there has been no systematic exploration of the spectrally resolved CLV. Here we make use of two spectral atlases recorded with the Fourier transform spectrometer (FTS) at the McMath-Pierce facility at Kitt Peak. One spectral atlas obtained 10 arcsec inside the solar limb was recorded in 1978-79 as part of the first survey of the Second Solar Spectrum, while the other atlas is the well used reference NSO/Kitt Peak FTS atlas for the disk center. Both atlases represent fully resolved spectra without any spectral stray light. We then construct an atlas of the limb/disk-center ratio between the two spectra over the wavelength range 4084-9950 Å. This ratio spectrum, which expresses the CLV amplitude relative to the continuum, is as richly structured as the intensity spectrum itself, but the line profiles differ greatly in both shape and amplitude. It is as if we are dealing with a new, unfamiliar spectrum of the Sun, distinctly different from both the intensity spectrum (which we here refer to with the acronym SS1) and the linear polarization of the Second Solar Spectrum (for which we use acronym SS2). In analogy we refer to the new ratio spectrum as SS3. While there is hardly any resemblance between SS3 and SS2, we are able to identify a non-linear mapping that can translate SS1 to SS3 in the case of weak to medium-strong spectral lines that are mainly formed in LTE (being directly coupled to the local temperature-density structure). This non-linear mapping is successfully modeled in terms of two free parameters that are found to vary approximately linearly over the entire wavelength range covered. These parameters and the various SS3 line profiles provide a novel, rich set of observational constraints, which may be used to test the validity of model atmospheres or guide the construction of improved models.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

### 2014

Steiner, O.; Salhab, R.; Freytag, B.; Rajaguru, P.; Schaffenberger, W.; Steffen, M.

Properties of small-scale magnetism of stellar atmospheres Journal Article

In: Publications of the ASJ, vol. 66, pp. S5, 2014.

@article{2014PASJ...66S...5S,

title = {Properties of small-scale magnetism of stellar atmospheres},

author = {Steiner, O. and Salhab, R. and Freytag, B. and Rajaguru, P. and Schaffenberger, W. and Steffen, M.},

url = {http://adsabs.harvard.edu/abs/2014PASJ...66S...5S},

doi = {10.1093/pasj/psu083},

year = {2014},

date = {2014-12-01},

urldate = {2014-12-01},

journal = {Publications of the ASJ},

volume = {66},

pages = {S5},

abstract = {The magnetic field outside of sunspots is concentrated in the intergranular space, where it forms a delicate filigree of bright ribbons and dots as seen on broad band images of the Sun. We expect this small-scale magnetic field to exhibit a similar behavior in stellar atmospheres. In order to find out more about it, we perform numerical simulations of the surface layers of stellar atmospheres. Here, we report on preliminary results from simulations in the range between 4000 K and 6500 K effective temperature with an initial vertical, homogeneous magnetic field of 50 G strength. We find that the field strength of the strongest magnetic flux concentrations increases with decreasing effective temperature at the height level where the average Rosseland optical depth is one. On the other hand, at the same level, the field is less strong than the thermal equipartition value in the coolest model but assumes superequipartition in the models hotter than 5000 K. While the Wilson depression of the strongest field concentrations is about one pressure scale height in the coolest model, it is more than four times the pressure scale height in the hottest one. We also find that the relative contribution of the bright filigree to the bolometric, vertically directed radiative intensity is most significant for the Teff = 5000 K model (0.6%-0.79%) and least significant for the hottest and coolest models (0.1%-0.46% and 0.14%-0.32%, respectively). This behavior suggests that the effect of the small-scale magnetic field on the photometric variability is more significant for K dwarf stars than for F-type and also M-type stars.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sowmya, K.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.

Polarized Light Scattering with the Paschen-Back Effect, Level-crossing of Fine Structure States, and Partial Frequency Redistribution Journal Article

In: Astrophysical Journal, vol. 793, pp. 71, 2014.

@article{2014ApJ...793...71S,

title = {Polarized Light Scattering with the Paschen-Back Effect, Level-crossing of Fine Structure States, and Partial Frequency Redistribution},

author = {Sowmya, K. and Nagendra, K.N. and Sampoorna, M. and Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2014ApJ...793...71S},

doi = {10.1088/0004-637X/793/2/71},

year = {2014},

date = {2014-10-01},

urldate = {2014-10-01},

journal = {Astrophysical Journal},

volume = {793},

pages = {71},

abstract = {The quantum interference between the fine structure states of an atom modifies the shapes of the emergent Stokes profiles in the second solar spectrum. This phenomenon has been studied in great detail both in the presence and absence of magnetic fields. By assuming a flat-spectrum for the incident radiation, the signatures of this effect have been explored for arbitrary field strengths. Even though the theory which takes into account the frequency dependence of the incident radiation is well developed, it is restricted to the regime in which the magnetic splitting is much smaller than the fine structure splitting. In the present paper, we carry out a generalization of our scattering matrix formalism including the effects of partial frequency redistribution for arbitrary magnetic fields. We test the formalism using available benchmarks for special cases. In particular, we apply it to the Li I 6708 Å D1 and D2 line system, for which observable effects from the Paschen-Back regime are expected in the Sun's spectrum.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.; Ramelli, R.; Ravindra, B.; Anusha, L. S.

Center-to-limb Observations and Modeling of the Ca I 4227 Å line Journal Article

In: Astrophysical Journal, vol. 793, pp. 42, 2014.

@article{2014ApJ...793...42S,

title = {Center-to-limb Observations and Modeling of the Ca I 4227 Å line},

author = {Supriya, H.D. and Smitha, H.N. and Nagendra, K.N. and Stenflo, J.O. and Bianda, M. and Ramelli, R. and Ravindra, B. and Anusha, L.S.},

url = {https://www.irsol.usi.ch/data/Papers/CaI_clv-final-submitted.pdf},

doi = {10.1088/0004-637X/793/1/42},

year = {2014},

date = {2014-09-01},

urldate = {2014-09-01},

journal = {Astrophysical Journal},

volume = {793},

pages = {42},

abstract = {The observed center-to-limb variation (CLV) of the scattering polarization in different lines of the Second Solar Spectrum can be used to constrain the height variation of various atmospheric parameters, in particular the magnetic fields, via the Hanle effect. Here we attempt to model the nonmagnetic CLV observations of the Q/I profiles of the Ca I 4227 Å line recorded with the Zurich Imaging Polarimeter-3 at IRSOL. For modeling, we use the polarized radiative transfer with partial frequency redistribution with a number of realistic one-dimensional (1D) model atmospheres. We find that all the standard Fontenla-Avrett-Loeser (FAL) model atmospheres, which we used, fail to simultaneously fit the observed (I, Q/I) at all the limb distances (μ). However, an attempt is made to find a single model which can provide a fit to at least the CLV of the observed Q/I instead of a simultaneous fit to the (I, Q/I) at all μ. To this end we construct a new 1D model by combining two of the standard models after modifying their temperature structures in the appropriate height ranges. This new combined model closely reproduces the observed Q/I at all μ but fails to reproduce the observed rest intensity at different μ. Hence we find that no single 1D model atmosphere succeeds in providing a good representation of the real Sun. This failure of 1D models does not, however, cause an impediment to the magnetic field diagnostic potential of the Ca I 4227 Å line. To demonstrate this we deduce the field strength at various μ positions without invoking the use of radiative transfer.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ishikawa, R.; Asensio Ramos, A.; Belluzzi, L.; Manso Sainz, R.; Stepan, J.; Trujillo Bueno, J.; Goto, M.; Tsuneta, S.

On the Inversion of the Scattering Polarization and the Hanle Effect Signals in the Hydrogen Ly-alpha Line Journal Article

In: Astrophysical Journal, vol. 787, pp. 159, 2014.

@article{2014ApJ...787..159I,

title = {On the Inversion of the Scattering Polarization and the Hanle Effect Signals in the Hydrogen Ly-alpha Line},

author = {Ishikawa, R. and Asensio Ramos, A. and Belluzzi, L. and Manso Sainz, R. and Stepan, J. and Trujillo Bueno, J. and Goto, M. and Tsuneta, S.},

url = {http://arxiv.org/pdf/1404.0786v1},

doi = {10.1088/0004-637X/787/2/159},

year = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Astrophysical Journal},

volume = {787},

pages = {159},

abstract = {Magnetic field measurements in the upper chromosphere and above, where the gas-to-magnetic pressure ratio β is lower than unity, are essential for understanding the thermal structure and dynamical activity of the solar atmosphere. Recent developments in the theory and numerical modeling of polarization in spectral lines have suggested that information on the magnetic field of the chromosphere-corona transition region could be obtained by measuring the linear polarization of the solar disk radiation at the core of the hydrogen Lyα line at 121.6 nm, which is produced by scattering processes and the Hanle effect. The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) sounding rocket experiment aims to measure the intensity (Stokes I) and the linear polarization profiles (Q/I and U/I) of the hydrogen Lyα line. In this paper, we clarify the information that the Hanle effect can provide by applying a Stokes inversion technique based on a database search. The database contains all theoretical Q/I and U/I profiles calculated in a one-dimensional semi-empirical model of the solar atmosphere for all possible values of the strength, inclination, and azimuth of the magnetic field vector, though this atmospheric region is highly inhomogeneous and dynamic. We focus on understanding the sensitivity of the inversion results to the noise and spectral resolution of the synthetic observations as well as the ambiguities and limitation inherent to the Hanle effect when only the hydrogen Lyα is used. We conclude that spectropolarimetric observations with CLASP can indeed be a suitable diagnostic tool for probing the magnetism of the transition region, especially when complemented with information on the magnetic field azimuth that can be obtained from other instruments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sowmya, K.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.

Polarized Scattering with Paschen-Back Effect, Hyperfine Structure, and Partial Frequency Redistribution in Magnetized Stellar Atmospheres Journal Article

In: Astrophysical Journal, vol. 786, pp. 150, 2014.

@article{2014ApJ...786..150S,

title = {Polarized Scattering with Paschen-Back Effect, Hyperfine Structure, and Partial Frequency Redistribution in Magnetized Stellar Atmospheres},

author = {Sowmya, K. and Nagendra, K.N. and Stenflo, J.O. and Sampoorna, M.},

url = {http://adsabs.harvard.edu/abs/2014ApJ...793...71S},

doi = {10.1088/0004-637X/786/2/150},

year = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Astrophysical Journal},

volume = {786},

pages = {150},

abstract = {F-state interference significantly modifies the polarization produced by scattering processes in the solar atmosphere. Its signature in the emergent Stokes spectrum in the absence of magnetic fields is depolarization in the line core. In the present paper, we derive the partial frequency redistribution (PRD) matrix that includes interference between the upper hyperfine structure states of a two-level atom in the presence of magnetic fields of arbitrary strengths. The theory is applied to the Na I D2 line that is produced by the transition between the lower J = 1/2 and upper J = 3/2 states which split into F states because of the coupling with the nuclear spin Is = 3/2. The properties of the PRD matrix for the single-scattering case is explored, in particular, the effects of the magnetic field in the Paschen-Back regime and their usefulness as a tool for the diagnostics of solar magnetic fields. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.; Ramelli, R.

The Quantum Interference Effects in the Sc II 4247 Å Line of the Second Solar Spectrum Journal Article

In: Astrophysical Journal, vol. 794, no. 1, pp. 30, 2014.

@article{0004-637X-794-1-30,

title = {The Quantum Interference Effects in the Sc II 4247 Å Line of the Second Solar Spectrum},

author = {Smitha, H.N. and Nagendra, K.N. and Stenflo, J.O. and Bianda, M. and Ramelli, R.},

url = {http://stacks.iop.org/0004-637X/794/i=1/a=30},

year = {2014},

date = {2014-01-01},

urldate = {2014-01-01},

journal = {Astrophysical Journal},

volume = {794},

number = {1},

pages = {30},

abstract = {The Sc II 4247 Å line formed in the chromosphere is one of the lines well known, like the Na I D2 and Ba II D2, for its prominent triple-peak structure in Q/I and the underlying quantum interference effects governing it. In this paper, we try to study the nature of this triple-peak structure using the theory of F-state interference including the effects of partial frequency redistribution (PRD) and radiative transfer (RT). We compare our results with the observations taken in a quiet region near the solar limb. In spite of accounting for PRD and RT effects, it has not been possible to reproduce the observed triple-peak structure in Q/I. While the two wing PRD peaks (on either side of central peak) and the near wing continuum can be reproduced, the central peak is completely suppressed by the enhanced depolarization resulting from the hyperfine structure splitting. This suppression remains for all the tested widely different one-dimensional model atmospheres or for any multi-component combinations of them. While multidimensional RT effects may improve the fit to the intensity profiles, they do not appear capable of explaining the enigmatic central Q/I peak. This leads us to suspect that some aspect of quantum physics is missing.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

### 2013

Stenflo, J. O.

Solar magnetic fields as revealed by Stokes polarimetry Journal Article

In: Astronomy and Astrophysics Reviews, vol. 21, pp. 66, 2013.

@article{2013A&ARv..21...66S,

title = {Solar magnetic fields as revealed by Stokes polarimetry},

author = {Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2013A%26ARv..21...66S},

doi = {10.1007/s00159-013-0066-3},

year = {2013},

date = {2013-09-01},

urldate = {2013-09-01},

journal = {Astronomy and Astrophysics Reviews},

volume = {21},

pages = {66},

abstract = {Observational astrophysics started when spectroscopy could be applied to astronomy. Similarly, observational work on stellar magnetic fields became possible with the application of spectro-polarimetry. In recent decades there have been dramatic advances in the observational tools for spectro-polarimetry. The four Stokes parameters that provide a complete representation of partially polarized light can now be simultaneously imaged with megapixel array detectors with high polarimetric precision (10-5 in the degree of polarization). This has led to new insights about the nature and properties of the magnetic field, and has helped pave the way for the use of the Hanle effect as a diagnostic tool beside the Zeeman effect. The magnetic structuring continues on scales orders of magnitudes smaller than the resolved ones, but various types of spectro-polarimetric signatures can be identified, which let us determine the field strengths and angular distributions of the field vectors in the spatially unresolved domain. Here we review the observational properties of the magnetic field, from the global patterns to the smallest scales at the magnetic diffusion limit, and relate them to the global and local dynamos.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.

Line-interlocking Effects on Polarization in Spectral Lines by Rayleigh and Raman Scattering Journal Article

In: Astrophysical Journal, vol. 770, pp. 92, 2013.

@article{2013ApJ...770...92S,

title = {Line-interlocking Effects on Polarization in Spectral Lines by Rayleigh and Raman Scattering},

author = {Sampoorna, M. and Nagendra, K.N. and Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2013ApJ...770...92S},

doi = {10.1088/0004-637X/770/2/92},

year = {2013},

date = {2013-06-01},

urldate = {2013-06-01},

journal = {Astrophysical Journal},

volume = {770},

pages = {92},

abstract = {The polarized spectrum of the Sun and stars is formed from the scattering of anisotropic radiation on atoms. Interpretation of this spectrum requires the solution of polarized line transfer in multilevel atomic systems. While sophisticated quantum theories of polarized line formation in multilevel atomic systems exist, they are limited by the approximation of complete frequency redistribution in scattering. The partial frequency redistribution (PRD) in line scattering is a necessary component in modeling the polarized spectra of strong lines. The polarized PRD line scattering theories developed so far confine themselves to a two-level or a two-term atom model. In this paper, we present a heuristic approach to the problem of polarized line formation in multilevel atoms taking into account the effects of PRD and a weak magnetic field. Starting from the unpolarized PRD multilevel atom approach of Hubeny et al., we incorporate the polarization state of the radiation field. However, the lower level polarization is neglected. Two iterative methods of solving the polarized PRD line transfer in multilevel atoms are also presented. Taking the example of a five-level Ca II atom model, we present illustrative results for an isothermal one-dimensional model atmosphere},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.

Modeling the Quantum Interference Signatures of the Ba II D2 4554 Å Line in the Second Solar Spectrum Journal Article

In: Astrophysical Journal, vol. 768, pp. 163, 2013.

@article{2013ApJ...768..163S,

title = {Modeling the Quantum Interference Signatures of the Ba II D2 4554 Å Line in the Second Solar Spectrum},

author = {Smitha, H.N. and Nagendra, K.N. and Stenflo, J.O. and Sampoorna, M.},

url = {http://adsabs.harvard.edu/abs/2013ApJ...768..163S},

doi = {10.1088/0004-637X/768/2/163},

year = {2013},

date = {2013-05-01},

urldate = {2013-05-01},

journal = {Astrophysical Journal},

volume = {768},

pages = {163},

abstract = {Quantum interference effects play a vital role in shaping the linear polarization profiles of solar spectral lines. The Ba II D2 line at 4554 Å is a prominent example, where the F-state interference effects due to the odd isotopes produce polarization profiles, which are very different from those of the even isotopes that have no F-state interference. It is therefore necessary to account for the contributions from the different isotopes to understand the observed linear polarization profiles of this line. Here we do radiative transfer modeling with partial frequency redistribution (PRD) of such observations while accounting for the interference effects and isotope composition. The Ba II D2 polarization profile is found to be strongly governed by the PRD mechanism. We show how a full PRD treatment succeeds in reproducing the observations, while complete frequency redistribution alone fails to produce polarization profiles that have any resemblance to the observed ones. However, we also find that the line center polarization is sensitive to the temperature structure of the model atmosphere. To obtain a good fit to the line center peak of the observed Stokes Q/I profile, a small modification of the FALX model atmosphere is needed, by lowering the temperature in the line-forming layers. Because of the pronounced temperature sensitivity of the Ba II D2 line it may not be a suitable tool for Hanle magnetic-field diagnostics of the solar chromosphere, because there is currently no straightforward way to separate the temperature and magnetic-field effects from each other.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Stenflo, J. O.; Demidov, M. L.; Bianda, M.; Ramelli, R.

Calibration of the 6302/6301 Stokes V line ratio in terms of the 5250/5247 ratio Journal Article

In: Astronomy and Astrophysics, vol. 556, pp. A113, 2013.

@article{2013A&A...556A.113S,

title = {Calibration of the 6302/6301 Stokes V line ratio in terms of the 5250/5247 ratio},

author = {Stenflo, J.O. and Demidov, M.L. and Bianda, M. and Ramelli, R.},

url = {https://www.irsol.usi.ch/data/Papers/AA556_A113_2013.pdf},

doi = {10.1051/0004-6361/201321749},

year = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {Astronomy and Astrophysics},

volume = {556},

pages = {A113},

abstract = {Four decades ago the Stokes V line ratio in the Fe i 5247.06 and 5250.22 Å lines was introduced as a powerful means of exploring the intrinsic field strengths at sub-pixel scales, which led to the discovery that most of the photospheric flux is in intermittent kG form. The "green" 5247-5250 line pair is unique because it allows the magnetic-field effects to be isolated from the thermodynamic effects. No other line pair with this property has since been identified. In recent years much of the magnetic-field diagnostics has been based on the "red" Fe i 6301.5 and 6302.5 Å line pair, since it was chosen in the design of the Hinode space observatory. Although thermodynamic effects severely contaminate the magnetic-field signatures for this line ratio, it is still possible to use it to extract information on intrinsic magnetic fields, but only after it has been "renormalized", since otherwise it produces fictitious, superstrong fields everywhere. In the present work we explore the joint behavior of these two line ratios to determine how the "contaminated" red line ratio can be translated into the corresponding green line ratio, which then allows for a direct interpretation in terms of intrinsic magnetic fields. Our observations are mainly based on recordings with the ZIMPOL-3 spectro-polarimeter at IRSOL in Locarno, Switzerland, complemented by data from the STOP telescope at the Sayan solar observatory (Irkutsk, Russia). The IRSOL observations are unique by allowing both the green and red line pairs to be recorded simultaneously on the same CCD sensor. We show how the line ratios depend on both the measured flux densities and on the heliocentric distance (the μ value on the solar disk), and finally derive the calibration function that enables the red line ratio to be translated to the green ratio for each μ value.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Stenflo, J. O.

Horizontal or vertical magnetic fields on the quiet Sun. Angular distributions and their height variations Journal Article

In: Astronomy and Astrophysics, vol. 555, pp. A132, 2013.

@article{2013A&A...555A.132S,

title = {Horizontal or vertical magnetic fields on the quiet Sun. Angular distributions and their height variations},

author = {Stenflo, J.O.},

url = {http://adsabs.harvard.edu/abs/2013A%26A...555A.132S},

doi = {10.1051/0004-6361/201321608},

year = {2013},

date = {2013-01-01},

urldate = {2013-01-01},

journal = {Astronomy and Astrophysics},

volume = {555},

pages = {A132},

abstract = {Different analyses of identical Hinode SOT/SP data of quiet-Sun magnetic fields have in the past led to contradictory answers to the question of whether the angular distribution of field vectors is preferentially horizontal or vertical. These answers have been obtained by combining the measured circular and linear polarizations in different ways to derive the field inclinations. A problem with these combinations is that the circular and linear polarizations scale with field strength in profoundly different ways. Here, we avoid these problems by using an entirely different approach that is based exclusively on the fundamental symmetry properties of the transverse Zeeman effect for observations away from the disk center without any dependence on the circular polarization. Systematic errors are suppressed by the application of a doubly differential technique with the 5247-5250 Å line pair for observations with the ZIMPOL-2 imaging polarimeter on the French THEMIS telescope on Tenerife. For the weakest, intranetwork-type magnetic fields, the angular distribution changes sign with the center-to-limb distance, being preferentially horizontal limbwards of μ (cosine of the heliocentric angle) = 0.2, while favoring the vertical direction inside this disk position. Since decreasing μ corresponds to increasing height of line formation, this finding implies that the intranetwork fields are more peaked around the vertical direction in the low to middle photosphere, while they are more horizontal in the upper photosphere. The angular distribution is however also found to become more vertical with increasing flux density. Thus, all facular points that we have observed have a strong preference for the vertical direction for all disk positions, including those all the way to the extreme limb. In terms of spatial averages weighted by the intrinsic magnetic energy density, these results are independent of telescope resolution},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

### 2012

Ramelli, R.; Stellmacher, G.; Wiehr, E.; Bianda, M.

Helium Emissions Observed in Ground-Based Spectra of Solar Prominences Journal Article

In: Solar Physics, vol. 281, pp. 697-706, 2012.

@article{2012SoPh..281..697R,

title = {Helium Emissions Observed in Ground-Based Spectra of Solar Prominences},

author = {Ramelli, R. and Stellmacher, G. and Wiehr, E. and Bianda, M.},

url = {http://www.springerlink.com/content/y176797417028056/},

doi = {10.1007/s11207-012-0118-2},

year = {2012},

date = {2012-12-01},

urldate = {2012-12-01},

journal = {Solar Physics},

volume = {281},

pages = {697-706},

abstract = {The only prominent line of singly ionized helium in the visible spectral range, He II 4686 Å, is observed together with the He I 5015 Å singlet and the He I 4471 Å triplet line in solar prominences. The Na D2 emission is used as a tracer for He II emissions which are sufficiently bright to exceed the noise level near 10−6 of the disk-center intensity. The prominences thus selected are characterized by small non-thermal line broadening and almost absent velocity shifts, yielding narrow line profiles without wiggles. The reduced widths [ΔλD/λ] of He II 4686 Å are 1.5 times broader than those of the He I 4471 Å triplet and 1.65 times broader than those of the He I 5015 Å singlet. This indicates that the He lines originate in a prominence-corona transition region with outwards increasing temperature.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.; Sampoorna, M.; Ramelli, R.; Anusha, L. S.

J-state interference signatures in the second solar spectrum. Modeling the Cr i triplet at 5204-5208 Å Journal Article

In: Astronomy and Astrophysics, vol. 541, pp. A24, 2012.

@article{2012A&A...541A..24S,

title = {J-state interference signatures in the second solar spectrum. Modeling the Cr i triplet at 5204-5208 Å},

author = {Smitha, H.N. and Nagendra, K.N. and Stenflo, J.O. and Bianda, M. and Sampoorna, M. and Ramelli, R. and Anusha, L.S.},

url = {http://adsabs.harvard.edu/abs/2012A%26A...541A..24S},

doi = {10.1051/0004-6361/201118466},

year = {2012},

date = {2012-05-01},

urldate = {2012-05-01},

journal = {Astronomy and Astrophysics},

volume = {541},

pages = {A24},

abstract = {The scattering polarization in the solar spectrum is traditionally modeled with each spectral line treated separately, but this is generally inadequate for multiplets where J-state interference plays a significant role. Through simultaneous observations of all the 3 lines of a Cr i triplet, combined with realistic radiative transfer modeling of the data, we show that it is necessary to include J-state interference consistently when modeling lines with partially interacting fine structure components. Polarized line formation theory that includes J-state interference effects together with partial frequency redistribution for a two-term atom is used to model the observations. Collisional frequency redistribution is also accounted for. We show that the resonance polarization in the Cr i triplet is strongly affected by the partial frequency redistribution effects in the line core and near wing peaks. The Cr i triplet is quite sensitive to the temperature structure of the photospheric layers. Our complete frequency redistribution calculations in semi-empirical models of the solar atmosphere cannot reproduce the observed near wing polarization or the cross-over of the Stokes Q/I line polarization about the continuum polarization level that is due to the J-state interference. When however partial frequency redistribution is included, a good fit to these features can be achieved. Further, to obtain a good fit to the far wings, a small temperature enhancement of the FALF model in the photospheric layers is necessary.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

### 2011

Kleint, L.; Shapiro, A. I.; Berdyugina, S. V.; Bianda, M.

Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2 molecule Journal Article

In: Astronomy and Astrophysics, vol. 536, pp. A47, 2011.

@article{2011A&A...536A..47K,

title = {Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2 molecule},

author = {Kleint, L. and Shapiro, A.I. and Berdyugina, S.V. and Bianda, M.},

url = {http://adsabs.harvard.edu/abs/2011A%26A...536A..47K},

doi = {10.1051/0004-6361/201015857},

year = {2011},

date = {2011-12-01},

urldate = {2011-12-01},

journal = {Astronomy and Astrophysics},

volume = {536},

pages = {A47},

abstract = {Context. Scattering polarization measurements contain a wealth of information that needs a thorough interpretation. This often requires accounting for the non-local origin of photons with different frequencies and at different limb positions. Currently, modeling scattering polarization in several molecular C2 lines simultaneously is only successful for lines with similar quantum numbers. More sophisticated models are needed to understand the dependence on quantum numbers and to reliably derive the strength of the turbulent magnetic fields using the differential Hanle effect.

Aims: We have developed a non-LTE analyzing technique for the C2 lines to determine the strength of turbulent magnetic fields and have applied it to observations obtained during our synoptic program at the Istituto Ricerche SOlari Locarno (IRSOL).

Methods: The influence of magnetic fields on scattering polarization can be interpreted differentially, i.e., by comparing several spectral lines within one spectral region. Through the application of the differential Hanle effect and non-LTE 1D radiative transfer, we are able to infer a magnetic field strength from the photospheric C2 lines around 5141 Å. Compared to previous models we include the effect of collisions and investigate their dependence on the total angular momentum number J.

Results: We carry out a detailed parameter study to investigate the influence of model parameters on the resulting scattering polarization. A good fit can now be obtained for spectral lines from different C2 triplets. For the 78 measurements obtained during the solar minimum in 2007-2009 we infer a mean magnetic field strength of 7.41 G with a standard deviation of 0.76 G.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Aims: We have developed a non-LTE analyzing technique for the C2 lines to determine the strength of turbulent magnetic fields and have applied it to observations obtained during our synoptic program at the Istituto Ricerche SOlari Locarno (IRSOL).

Methods: The influence of magnetic fields on scattering polarization can be interpreted differentially, i.e., by comparing several spectral lines within one spectral region. Through the application of the differential Hanle effect and non-LTE 1D radiative transfer, we are able to infer a magnetic field strength from the photospheric C2 lines around 5141 Å. Compared to previous models we include the effect of collisions and investigate their dependence on the total angular momentum number J.

Results: We carry out a detailed parameter study to investigate the influence of model parameters on the resulting scattering polarization. A good fit can now be obtained for spectral lines from different C2 triplets. For the 78 measurements obtained during the solar minimum in 2007-2009 we infer a mean magnetic field strength of 7.41 G with a standard deviation of 0.76 G.

Anusha, L. S.; Nagendra, K. N.; Bianda, M.; Stenflo, J. O.; Holzreuter, R.; Sampoorna, M.; Frisch, H.; Ramelli, R.; Smitha, H. N.

Analysis of the Forward-scattering Hanle Effect in the Ca I 4227 Å Line Journal Article

In: Astrophysical Journal, vol. 737, pp. 95, 2011.

@article{2011ApJ...737...95A,

title = {Analysis of the Forward-scattering Hanle Effect in the Ca I 4227 Å Line},

author = {Anusha, L.S. and Nagendra, K.N. and Bianda, M. and Stenflo, J.O. and Holzreuter, R. and Sampoorna, M. and Frisch, H. and Ramelli, R. and Smitha, H.N.},

url = {https://www.irsol.usi.ch/data/Papers/anusha2011.pdf},

doi = {10.1088/0004-637X/737/2/95},

year = {2011},

date = {2011-08-01},

urldate = {2011-08-01},

journal = {Astrophysical Journal},

volume = {737},

pages = {95},

abstract = {Coherent scattering of limb-darkened radiation is responsible for the generation of the linearly polarized spectrum of the Sun (the Second Solar Spectrum). This Second Solar Spectrum is usually observed near the limb of the Sun, where the polarization amplitudes are largest. At the center of the solar disk the linear polarization is zero for an axially symmetric atmosphere. Any mechanism that breaks the axial symmetry (like the presence of an oriented magnetic field, or resolved inhomogeneities in the atmosphere) can generate a non-zero linear polarization. In the present paper we study the linear polarization near the disk center in a weakly magnetized region, where the axisymmetry is broken. We present polarimetric (I, Q/I, U/I, and V/I) observations of the Ca I 4227 Å line recorded around μ = cos θ = 0.9 (where θ is the heliocentric angle) and a modeling of these observations. The high sensitivity of the instrument (ZIMPOL-3) makes it possible to measure the weak polarimetric signals with great accuracy. The modeling of these high-quality observations requires the solution of the polarized radiative transfer equation in the presence of a magnetic field. For this we use standard one-dimensional model atmospheres. We show that the linear polarization is mainly produced by the Hanle effect (rather than by the transverse Zeeman effect), while the circular polarization is due to the longitudinal Zeeman effect. A unique determination of the full bm {B} vector may be achieved when both effects are accounted for. The field strengths required for the simultaneous fitting of Q/I, U/I, and V/I are in the range 10-50 G. The shapes and signs of the Q/I and U/I profiles are highly sensitive to the orientation of the magnetic field.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Bianda, M.; Ramelli, R.; Anusha, L. S.; Stenflo, J. O.; Nagendra, K. N.; Holzreuter, R.; Sampoorna, M.; Frisch, H.; Smitha, H. N.

Observations of the forward scattering Hanle Effect the Ca I 4227 Å Line Journal Article

In: Astronomy and Astrophysics, vol. 530, pp. L13, 2011.

@article{2011A&A...530L..13B,

title = {Observations of the forward scattering Hanle Effect the Ca I 4227 Å Line},

author = {Bianda, M. and Ramelli, R. and Anusha, L.S. and Stenflo, J.O. and Nagendra, K.N. and Holzreuter, R. and Sampoorna, M. and Frisch, H. and Smitha, H.N.},

url = {http://arxiv.org/abs/1105.2157},

doi = {10.1051/0004-6361/201117047},

year = {2011},

date = {2011-06-01},

urldate = {2011-06-01},

journal = {Astronomy and Astrophysics},

volume = {530},

pages = {L13},

abstract = {Chromospheric magnetic fields are notoriously difficult to measure. The chromospheric lines are broad, while the fields are producing a minuscule Zeeman-effect polarization. A promising diagnostic alternative is provided by the forward-scattering Hanle effect, which can be recorded in chromospheric lines such as the He i 10 830 Å and the Ca i 4227 Å lines. We present a set of spectropolarimetric observations of the full Stokes vector obtained near the center of the solar disk in the Ca i 4227 Å line with the ZIMPOL polarimeter at the IRSOL observatory. We detect a number of interesting forward-scattering Hanle effect signatures, which we model successfully using polarized radiative transfer. Here we focus on the observational aspects, while a separate companion paper deals with the theoretical modeling.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Bianda, M.; Ramelli, R.

NLTE modeling of Stokes vector center-to-limb variations in the CN violet system Journal Article

In: Astronomy and Astrophysics, vol. 529, pp. A139, 2011.

@article{2011A&A...529A.139S,

title = {NLTE modeling of Stokes vector center-to-limb variations in the CN violet system},

author = {Shapiro, A.I. and Fluri, D.M. and Berdyugina, S.V. and Bianda, M. and Ramelli, R.},

url = {https://www.irsol.usi.ch/data/Papers/aa11299-08.pdf},

doi = {10.1051/0004-6361/200811299},

year = {2011},

date = {2011-05-01},

urldate = {2011-05-01},

journal = {Astronomy and Astrophysics},

volume = {529},

pages = {A139},

abstract = {Context. The solar surface magnetic field is connected with and even controls most of the solar activity phenomena. Zeeman effect diagnostics allow for measuring only a small fraction of the fractal-like structured magnetic field. The remaining hidden magnetic fields can only be accessed with the Hanle effect.

Aims: Molecular lines are very convenient for applying the Hanle effect diagnostics thanks to the broad range of magnetic sensitivities in a narrow spectral region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno (IRSOL), we simultaneously observed intensity and linear polarization center-to-limb variations in two spectral regions containing the (0, 0) and (1, 1) bandheads of the CN B2Σ - X2Σ system. Here we present an analysis of these observations.

Methods: We have implemented coherent scattering in molecular lines into an NLTE radiative transfer code. A two-step approach was used. First, we separately solved the statistical equilibrium equations and compute opacities and intensity while neglecting polarization. Then we used these quantities as input for calculating scattering polarization and the Hanle effect.

Results: We have found that it is impossible to fit the intensity and polarization simultaneously at different limb angles in the framework of standard 1D modeling. The atmosphere models that provide correct intensity center-to-limb variations fail to fit linear polarization center-to-limb variations due to lacking radiation-field anisotropy. We had to increase the anisotropy by means of a specially introduced free parameter. This allows us to successfully interpret our observations. We discuss possible reasons for underestimating the anisotropy in the 1D modeling. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Aims: Molecular lines are very convenient for applying the Hanle effect diagnostics thanks to the broad range of magnetic sensitivities in a narrow spectral region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno (IRSOL), we simultaneously observed intensity and linear polarization center-to-limb variations in two spectral regions containing the (0, 0) and (1, 1) bandheads of the CN B2Σ - X2Σ system. Here we present an analysis of these observations.

Methods: We have implemented coherent scattering in molecular lines into an NLTE radiative transfer code. A two-step approach was used. First, we separately solved the statistical equilibrium equations and compute opacities and intensity while neglecting polarization. Then we used these quantities as input for calculating scattering polarization and the Hanle effect.

Results: We have found that it is impossible to fit the intensity and polarization simultaneously at different limb angles in the framework of standard 1D modeling. The atmosphere models that provide correct intensity center-to-limb variations fail to fit linear polarization center-to-limb variations due to lacking radiation-field anisotropy. We had to increase the anisotropy by means of a specially introduced free parameter. This allows us to successfully interpret our observations. We discuss possible reasons for underestimating the anisotropy in the 1D modeling.

### 2010

Kleint, L.; Berdyugina, S. V.; Shapiro, A. I.; Bianda, M.

Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum Journal Article

In: Astronomy and Astrophysics, vol. 524, pp. A37, 2010.

@article{2010A&A...524A..37K,

title = {Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum},

author = {Kleint, L. and Berdyugina, S.V. and Shapiro, A.I. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/aa15285-10.pdf},

doi = {10.1051/0004-6361/201015285},

year = {2010},

date = {2010-12-01},

urldate = {2010-12-01},

journal = {Astronomy and Astrophysics},

volume = {524},

pages = {A37},

abstract = {Context. Small-scale, weak magnetic fields are ubiquitous in the quiet solar atmosphere. Yet their properties and temporal and spatial variations are not well known.

Aims: We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle.

Methods: Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5” on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of ~10-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C2 triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C2 R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields.

Results: We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3-8 G.

Conclusions: Our results demonstrate the potential of long-term observations of small-scale magnetic fields, which may vary with the solar cycle in both mean strength and spatial distribution. This provides important constraints on the energy budget of the solar cycle. Extending this synoptic program to many spectral lines would provide a sample of heights in the solar atmosphere. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Aims: We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle.

Methods: Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5” on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of ~10-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C2 triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C2 R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields.

Results: We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3-8 G.

Conclusions: Our results demonstrate the potential of long-term observations of small-scale magnetic fields, which may vary with the solar cycle in both mean strength and spatial distribution. This provides important constraints on the energy budget of the solar cycle. Extending this synoptic program to many spectral lines would provide a sample of heights in the solar atmosphere.

Anusha, L. S.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.; Sampoorna, M.; Frisch, H.; Holzreuter, R.; Ramelli, R.

Generalization of the Last Scattering Approximation for the Second Solar Spectrum Modeling: The Ca I 4227 Å Line as a Case Study Journal Article

In: Astrophysical Journal, vol. 718, pp. 988-1000, 2010.

@article{2010ApJ...718..988A,

title = {Generalization of the Last Scattering Approximation for the Second Solar Spectrum Modeling: The Ca I 4227 Å Line as a Case Study},

author = {Anusha, L.S. and Nagendra, K.N. and Stenflo, J.O. and Bianda, M. and Sampoorna, M. and Frisch, H. and Holzreuter, R. and Ramelli, R.},

url = {https://www.irsol.usi.ch/data/Papers/anusha10.pdf},

doi = {10.1088/0004-637X/718/2/988},

year = {2010},

date = {2010-08-01},

urldate = {2010-08-01},

journal = {Astrophysical Journal},

volume = {718},

pages = {988-1000},

abstract = {To model the second solar spectrum (the linearly polarized spectrum of the Sun that is due to coherent scattering processes), one needs to solve the polarized radiative transfer (RT) equation. For strong resonance lines, partial frequency redistribution (PRD) effects must be accounted for, which make the problem computationally demanding. The "last scattering approximation" (LSA) is a concept that has been introduced to make this highly complex problem more tractable. An earlier application of a simple LSA version could successfully model the wings of the strong Ca I 4227 Å resonance line in Stokes Q/I (fractional linear polarization), but completely failed to reproduce the observed Q/I peak in the line core. Since the magnetic field signatures from the Hanle effect only occur in the line core, we need to generalize the existing LSA approach if it is to be useful for the diagnostics of chromospheric and turbulent magnetic fields. In this paper, we explore three different approximation levels for LSA and compare each of them with the benchmark represented by the solution of the full polarized RT, including PRD effects. The simplest approximation level is LSA-1, which uses the observed center-to-limb variation of the intensity profile to obtain the anisotropy of the radiation field at the surface, without solving any transfer equation. In contrast, the next two approximation levels use the solution of the unpolarized transfer equation to derive the anisotropy of the incident radiation field and use it as an input. In the case of LSA-2, the anisotropy at level τλ = μ, the atmospheric level from which an observed photon is most likely to originate, is used. LSA-3, on the other hand, makes use of the full depth dependence of the radiation anisotropy. The Q/I formula for LSA-3 is obtained by keeping the first term in a series expansion of the Q-source function in powers of the mean number of scattering events. Computationally, LSA-1 is 21 times faster than LSA-2, which is 5 times faster than the more general LSA-3, which itself is 8 times faster than the polarized RT approach. A comparison of the calculated Q/I spectra with the RT benchmark shows excellent agreement for LSA-3, including good modeling of the Q/I core region with its PRD effects. In contrast, both LSA-1 and LSA-2 fail to model the core region. The RT and LSA-3 approaches are then applied to model the recently observed Q/I profile of the Ca I 4227 Å line in quiet regions of the Sun. Apart from a global scale factor both give a very good fit to the Q/I spectra for all the wavelengths, including the core peak and blend line depolarizations. We conclude that LSA-3 is an excellent substitute for the full polarized RT and can be used to interpret the second solar spectrum, including the Hanle effect with PRD. It also allows the techniques developed for unpolarized three-dimensional RT to be applied to the modeling of the second solar spectrum. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}