Refereed Scientific Papers

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@article{2041-8205-711-2-L57,

title = {Statistical Analysis of the very Quiet Sun Magnetism},

author = {Martínez González, M.J. and Manso Sainz, R. and Asensio Ramos, A. and López Ariste, A. and Bianda, M.},

url = {http://stacks.iop.org/2041-8205/711/i=2/a=L57},

year  = {2010},

date = {2010-01-01},

journal = {The Astrophysical Journal Letters},

volume = {711},

number = {2},

pages = {L57},

abstract = {The behavior of the observed polarization amplitudes with spatial resolution is a strong constraint on the nature and organization of solar magnetic fields below the resolution limit. We study the polarization of the very quiet Sun at different spatial resolutions using ground- and space-based observations. It is shown that 80% of the observed polarization signals do not change with spatial resolution, suggesting that, observationally, the very quiet Sun magnetism remains the same despite the high spatial resolution of space-based observations. Our analysis also reveals a cascade of spatial scales for the magnetic field within the resolution element. It is manifest that the Zeeman effect is sensitive to the microturbulent field usually associated with Hanle diagnostics. This demonstrates that Zeeman and Hanle studies show complementary perspectives of the same magnetism.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2009ApJ...699.1650S,

title = {Origin of Spatial Variations of Scattering Polarization in the Wings of the Ca I 4227 Å line},

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

url = {https://www.irsol.usi.ch/data/Papers/apj_699_2_1650.pdf},

doi = {10.1088/0004-637X/699/2/1650},

year  = {2009},

date = {2009-07-01},

urldate = {2009-07-01},

journal = {Astrophysical Journal},

volume = {699},

pages = {1650-1659},

abstract = {Polarization that is produced by coherent scattering can be modified by magnetic fields via the Hanle effect. This has opened a window to explorations of solar magnetism in parameter domains not accessible to the Zeeman effect. According to standard theory the Hanle effect should only be operating in the Doppler core of spectral lines but not in the wings. In contrast, our observations of the scattering polarization in the Ca I 4227 Å line reveal the existence of spatial variations of the scattering polarization throughout the far line wings. This raises the question whether the observed spatial variations in wing polarization have a magnetic or nonmagnetic origin. A magnetic origin may be possible if elastic collisions are able to cause sufficient frequency redistribution to make the Hanle effect effective in the wings without causing excessive collisional depolarization, as suggested by recent theories for partial frequency redistribution (PRD) with coherent scattering in magnetic fields. To model the wing polarization we bypass the problem of solving the full polarized radiative transfer equations and instead apply an extended version of the technique based on the "last scattering approximation." It assumes that the polarization of the emergent radiation is determined by the anisotropy of the incident radiation field at the last scattering event. We determine this anisotropy from the observed limb darkening as a function of wavelength throughout the spectral line. The empirical anisotropy profile is used together with the single-scattering redistribution matrix, which contains all the PRD, collisional, and magnetic field effects. The model further contains a continuum opacity parameter, which increasingly dilutes the polarized line photons as we move away from the line center, and a continuum polarization parameter that represents the observed polarization level far from the line. This model is highly successful in reproducing the observed Stokes Q/I polarization (linear polarization parallel to the nearest solar limb), including the location of the wing polarization maxima and the minima around the Doppler core, but it fails to reproduce the observed spatial variations of the wing polarization in terms of magnetic field effects with frequency redistribution. This null result points in the direction of a nonmagnetic origin in terms of local inhomogeneities (varying collisional depolarization, radiation-field anisotropies, and deviations from a plane-parallel atmospheric stratification).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2009A&A...506.1415B,

title = {The quiet Sun magnetic field observed with ZIMPOL on THEMIS. I. The probability density function},

author = {Bommier, V. and Martínez González, M.J. and Bianda, M. and Frisch, H. and Asensio Ramos, A. and Gelly, B. and Landi Degl'Innocenti, E.},

url = {https://www.irsol.usi.ch/data/Papers/bommier09.pdf},

doi = {10.1051/0004-6361/200811373},

year  = {2009},

date = {2009-01-01},

urldate = {2009-01-01},

journal = {Astronomy and Astrophysics},

volume = {506},

pages = {1415-1428},

abstract = {Context: The quiet Sun magnetic field probability density function (PDF) remains poorly known. Modeling this field also introduces a magnetic filling factor that is also poorly known. With these two quantities, PDF and filling factor, the statistical description of the quiet Sun magnetic field is complex and needs to be clarified.

Aims: In the present paper, we propose a procedure that combines direct determinations and inversion results to derive the magnetic field vector and filling factor, and their PDFs.

Methods: We used spectro-polarimetric observations taken with the ZIMPOL polarimeter mounted on the THEMIS telescope. The target was a quiet region at disk center. We analyzed the data by means of the UNNOFIT inversion code, with which we inferred the distribution of the mean magnetic field α B, α being the magnetic filling factor. The distribution of α was derived by an independent method, directly from the spectro-polarimetric data. The magnetic field PDF p(B) could then be inferred. By introducing a joint PDF for the filling factor and the magnetic field strength, we have clarified the definition of the PDF of the quiet Sun magnetic field when the latter is assumed not to be volume-filling.

Results: The most frequent local average magnetic field strength is found to be 13 G. We find that the magnetic filling factor is related to the magnetic field strength by the simple law α = B_1/B with B1 = 15 G. This result is compatible with the Hanle weak-field determinations, as well as with the stronger field determinations from the Zeeman effect (kGauss field filling 1-2% of space). From linear fits, we obtain the analytical dependence of the magnetic field PDF. Our analysis has also revealed that the magnetic field in the quiet Sun is isotropically distributed in direction.

Conclusions: We conclude that the quiet Sun is a complex medium where magnetic fields having different field strengths and filling factors coexist. Further observations with a better polarimetric accuracy are, however, needed to confirm the results obtained in the present work.

Based on observations made with the French-Italian telescope THEMIS operated by the CNRS and CNR on the island of Tenerife in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Present address: Instituto de Astrofísica de Canarias, vía Láctea s/n, 38205 La Laguna, Tenerife, Spain},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2007A&A...473L...1A,

title = {First polarimetric observations and modeling of the FeH F4Δ-X4Δ system},

author = {Afram, N and Berdyugina, S.V. and Fluri, D.M. and Semel, M. and Bianda, M. and Ramelli, R.},

url = {https://www.irsol.usi.ch/data/Papers/aa8109-07.pdf},

doi = {10.1051/0004-6361:20078109},

year  = {2007},

date = {2007-10-01},

urldate = {2007-10-01},

journal = {Astronomy and Astrophysics},

volume = {473},

pages = {L1-L4},

abstract = {Context: Lines of diatomic molecules are typically much more temperature and pressure sensitive than atomic lines, which makes them ideal, complementary tools for studying cool stellar atmospheres as well as the internal structure of sunspots and starspots. The FeH F^4Δ-X^4Δ system represents such an example that exhibits in addition a large magnetic field sensitivity. However, the current theoretical descriptions of these transitions including the molecular constants involved are only based on intensity measurements because polarimetric observations have not been available so far, which limits their diagnostic value. Furthermore, the theory was optimized to reproduce energy levels and line strengths without taking the magnetic sensitivities into account.

Aims: We present for the first time spectropolarimetric observations of the FeH F^4Δ-X^4Δ system measured in sunspots to investigate their diagnostic capabilities for probing solar and stellar magnetic fields. In particular, we investigate whether the current theoretical model of FeH can reproduce the observed Stokes profiles including their magnetic properties.

Methods: The polarimetric observations of the FeH F^4Δ-X^4Δ system in Stokes I and V are compared with synthetic Stokes profiles modeled with radiative transfer calculations. This allows us to infer the temperature and the magnetic field strength of the observed sunspots.

Results: We find that the current theory successfully reproduces the magnetic properties of a large number of lines in the FeH F^4Δ-X^4Δ system. In a few cases the observations indicate a larger Zeeman splitting than predicted by the theory. There, our observations have provided additional constraints, which allowed us to determine empirical molecular constants.

Conclusions: The FeH F^4Δ-X^4Δ system is found to be a very sensitive magnetic diagnostic tool. Polarimetric data of these lines, in contrast to intensity measurements, provide us with more direct and detailed information to study the coolest parts of sunspot and starspot umbrae, and cool active dwarfs},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2006ApJ...649L..49B,

title = {First Polarimetric Measurements and Modeling of the Paschen-Back Effect in CaH Transitions},

author = {Berdyugina, S.V. and Fluri, D.M. and Ramelli, R. and Bianda, M. and Gisler, D. and Stenflo, J.O.},

url = {https://www.irsol.usi.ch/data/Papers/berdyugina06.pdf},

doi = {10.1086/508159},

year  = {2006},

date = {2006-09-01},

urldate = {2006-09-01},

journal = {Astrophysical Journal, Letters},

volume = {649},

pages = {L49-L52},

abstract = {We report the first spectropolarimetric observations and modeling of CaH transitions in sunspots. We have detected strong polarization signals in many CaH lines from the A-X system, and we provide the first successful fit to the observed Stokes profiles using the previously developed theory of the Paschen-Back effect in arbitrary electronic states of diatomic molecules and polarized radiative transfer in molecular lines in stellar atmospheres. We analyze the CaH Stokes profiles together with quasi-simultaneous observations in TiO bands and conclude that CaH provides a valuable diagnostic of magnetic fields in sunspots, starspots, cool stars, and brown dwarfs},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2005A&A...434.1183B,

title = {Absence of linear polarization in Hα emission of solar flares},

author = {Bianda, M. and Benz, A.O. and Stenflo, J.O. and Küveler, G. and Ramelli, R.},

url = {https://www.irsol.usi.ch/data/Papers/aa1654.pdf},

doi = {10.1051/0004-6361:20041654},

year  = {2005},

date = {2005-05-01},

urldate = {2005-05-01},

journal = {Astronomy and Astrophysics},

volume = {434},

pages = {1183-1189},

abstract = {High sensitivity observations of Hα polarization of 30 flares of different sizes and disk positions are reported. Both filter and spectrographic techniques have been used. The ZIMPOL system eliminates spurious polarizations due to seeing and flat-field effects. We didn't find any clear linear polarization signature above our sensitivity level which was usually better than 0.1%. The observations include an X17.1 flare with gamma-ray lines reported by the RHESSI satellite. These results cast serious doubts on previous claims of linear polarization at the one percent level and more, attributed to impact polarization. The absence of linear polarization limits the anisotropy of energetic protons in the Hα emitting region. The likely causes are isotropization by collisions with neutrals in the chromosphere and defocusing by the converging magnetic field. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2004ApJ...611L..61A,

title = {Observation of the Molecular Zeeman Effect in the G Band},

author = {Asensio Ramos, A. and Trujillo Bueno, J. and Bianda, M. and Manso Sainz, R. and Uitenbroek, H.},

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

doi = {10.1086/423617},

year  = {2004},

date = {2004-08-01},

urldate = {2004-08-01},

journal = {Astrophysical Journal, Letters},

volume = {611},

pages = {L61-L64},

abstract = {Here we report on the first observational investigation of the Zeeman effect in the G band around 4305 Å. Our spectropolarimetric observations of sunspots with the Zürich Imaging Polarimeter at the Istituto Ricerche Solari Locarno confirm our previous theoretical prediction that the molecular Zeeman effect produces measurable circular polarization signatures in several CH lines that are not overlapped with atomic transitions. We also find both circular and linear polarization signals produced by atomic lines whose wavelengths lie in the G-band spectral region. Together, such molecular and atomic lines are potentially important for empirical investigations of solar and stellar magnetism. For instance, a comparison between observed and calculated Stokes profiles suggests that the thermodynamical and/or magnetic properties of the photospheric regions of sunspot umbrae are horizontally structured with a component that might be associated with umbral dots.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2003A&A...404L..25W,

title = {Solar prominence polarimetry},

author = {Wiehr, E. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/Wiehr03.pdf},

doi = {10.1051/0004-6361:20030668},

year  = {2003},

date = {2003-06-01},

urldate = {2003-06-01},

journal = {Astronomy and Astrophysics},

volume = {404},

pages = {L25-L28},

abstract = {We measure the resonance polarization in solar prominences in Hα , Hβ and HeD3. A two-dimensional set-up with narrow-band filter, polarization analyzer and CCD camera is used to take prominence images in polarized light at high spatial resolution. Placed on a coudé telescope's hour axis, the observations near the equinoxia are free from purely instrumental polarization. Above the 0.1% noise limit, the Balmer lines do not show a polarization in contrast to the HeD3 line. Here, we determine the complete polarization profile after exchange of filter and CCD with the spectrograph, keeping the polarization analyzer fixed. In most prominences the Stokes-U and -Q profiles are not similar to Stokes-I: occasionally the blue and the red components of the emission are equal or even show a reverse ratio. This fits calculations for magnetic field strengths of the order of 50 Gauß being markedly stronger than commonly assumed. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2003A&A...398..739W,

title = {High spatial resolution solar polarimetry with interference filters},

author = {Wiehr, E. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/AA398-739.pdf},

doi = {10.1051/0004-6361:20021515},

year  = {2003},

date = {2003-02-01},

urldate = {2003-02-01},

journal = {Astronomy and Astrophysics},

volume = {398},

pages = {739-742},

abstract = {A new type of two-dimensional polarimeter is used to measure the center-to-limb variation of the scattering induced polarization in a narrow continuum window up to the extreme limb. The polarimeter is set on the Tenerife Gregory Coudé telescope's hour axis, where the two folding flat mirrors cancel their polarizing effects for zero solar declination at the equinox. The short CCD exposure of only 5 ms allows high spatial resolution images in polarized light. A beam switching technique together with an integration parallel to the solar limb over 20arcsec, yields a high polarimetric accuracy with an rms noise of 2*E-4. Our results for a continuum window at 4506-4508 Å agree with model calculations down to limb distances of 0farcs32 (i.e. cos vartheta < 0.025)},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2002A&A...389..314S,

title = {Spatial mapping of the Hanle and Zeeman effects on the Sun},

author = {Stenflo, J.O. and Gandorfer, A. and Holzreuter, R. and Gisler, D. and Keller, C.U. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/AA389_314_2002.pdf},

doi = {10.1051/0004-6361:20020580},

year  = {2002},

date = {2002-07-01},

urldate = {2002-07-01},

journal = {Astronomy and Astrophysics},

volume = {389},

pages = {314-324},

abstract = {Spatial mapping of the Hanle and Zeeman effects on the Sun has been done for the first time, through Stokes vector imaging with a narrow-band (0.2 Å) universal filter. It is shown how the polarization signatures of the Hanle and Zeeman effects can be cleanly distinguished from each other by comparing the Stokes images recorded at different, specially selected wavelengths within the Na I D2-D1 line system. Examples of the polarization signatures of sunspots, faculae, the supergranulation network, and large-scale canopy fields are shown. The most striking result of our observations is that the scattering polarization has an extremely intermittent structure rather than being a simple function of limb distance. These intermittent scattering polarization signals are cospatial with the facular and supergranulation network seen both in intensity and circular polarization. The observed pattern can be explained in terms of magnetic enhancement of the scattering polarization in the network and/or Hanle depolarization of the scattering polarization outside the network. Since however no magnetic fields are seen in circular polarization outside the network, the relative absence of linear scattering polarization there may be explained by Hanle depolarization only if the volume filling, depolarizing magnetic field has mixed polarities on a subarcsec scale that is not resolved. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{2002AAS...200.5907D,

title = {Fast Flat Fields from Moving Extended Sources},

author = {Dalrymple, N.E. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/Flatfield_2003.pdf},

year  = {2002},

date = {2002-05-01},

urldate = {2002-05-01},

booktitle = {American Astronomical Society Meeting Abstracts #200},

volume = {34},

pages = {741},

series = {Bulletin of the American Astronomical Society},

abstract = { The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1999A&A...350.1060B,

title = {Hanle effect observations with the CaI 4227 Å line},

author = {Bianda, M. and Stenflo, J.O. and Solanki, S.K.},

url = {https://www.irsol.usi.ch/data/Papers/AA350_1060_1999.pdf},

year  = {1999},

date = {1999-10-01},

urldate = {1999-10-01},

journal = {Astronomy and Astrophysics},

volume = {350},

pages = {1060-1070},

abstract = {The Hanle effect in the Ca i 4227 Ä line has been explored through the analysis of a large number of Stokes profile recordings obtained on the quiet Sun with the beam-splitter polarimeter system at IRSOL (Istituto Ricerche Solari Locarno). In contrast to previous Hanle observations with this line, which were limited to the Stokes I and Q parameters, we are now in a position to study the combined effects of Hanle depolarization (via Stokes Q) and rotation of the plane of linear polarization (via Stokes U) with the same methods that we recently applied to the Sr ii 4078 Ä line. The Hanle histograms for the distributions of the depolarization and rotation parameters are very similar for the two lines and show that there must be mixed contributions to the Hanle signals from spatially unresolved magnetic fields with random orientations (which do not contribute to Stokes U) and partially resolved magnetic fields with a net orientation of the field vectors. Field strengths in the range 5-10 G are preferred. We also determine the ``Hanle efficiency profile'', which shows how the Hanle effect is confined to the Doppler core but vanishes in the line wings. It is wider than the corresponding profile for the Sr line, as expected from the difference in atomic weight and wavelength between the two lines. The Q/I profiles of the Ca i 4227 Ä line have minima around the Doppler core which turn negative (polarization perpendicular to the limb) for limb distances mu =cos theta >~ 0.2, a likely signature of partial redistribution effects},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1998A&A...337..565B,

title = {Hanle diagnostics of solar magnetic fields: the SrII 4078 Å line},

author = {Bianda, M. and Stenflo, J.O. and Solanki, S.K.},

url = {https://www.irsol.usi.ch/data/Papers/AA337_565_1998.pdf},

year  = {1998},

date = {1998-09-01},

urldate = {1998-09-01},

journal = {Astronomy and Astrophysics},

volume = {337},

pages = {565-578},

abstract = {The Hanle depolarization and rotation effects in the Sr ii 4078tsAngstroms line have been explored with the instrumentation at IRSOL (Istituto Ricerche Solari Locarno) by recording the Stokes I, Q, and U line profiles with high spectral resolution and polarimetric accuracy in a large number of regions across the solar disk. >From the extracted line parameters we have constructed ``Hanle histograms'' showing the statistical distributions of the Hanle rotation and depolarization effects. Comparison with theoretical calculations allow these histograms to be understood in terms of magnetic fields with a strength of about 5-10ts G, which is similar to the field strengths previously found through analysis of Q/I Hanle depolarization in the Ca i 4227tsAngstroms line. While small-scale magnetic fields with spatially unresolved angular distributions contribute to the observed Hanle depolarization effects, the observed Hanle rotation effects in Stokes U are due to spatially resolved fields with net large-scale orientations (e.g. global or canopy-type fields). We have also for the first time determined empirical ``Hanle efficiency profiles'', derived independently for the Hanle rotation and depolarization effects. They show how the Hanle efficiency has its maximum in the Doppler core of the line and then rapidly decreases to become zero in the line wings. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1998A&A...331..760B,

title = {Hanle depolarisation in the solar chromosphere},

author = {Bianda, M. and Solanki, S.K. and Stenflo, J.O.},

url = {https://www.irsol.usi.ch/data/Papers/AA331_760_1998.pdf},

year  = {1998},

date = {1998-03-01},

urldate = {1998-03-01},

journal = {Astronomy and Astrophysics},

volume = {331},

pages = {760-770},

abstract = {A new polarimeter and an extension of the polarimetric technique of Donati et al. (1990) to higher polarisation values are presented. With this system, which allows low-noise polarimetric observations in the blue and near-UV part of the solar spectrum, we have recorded more than 200 Q/I profiles of Ca I 4227 Angstroms with an accuracy of 2-3x 10(-4) . We find that while the Q/I line shape outside the line core is very stable from one part of the solar surface to the next, the line core exhibits large variations relative to the rest of the profile. These variations are best interpreted in terms of partial depolarisation produced by the Hanle effect in the presence of weak fields. We obtain a quantitative and model-independent estimate of the depolarisation, from which we deduce field strengths of 5-15 G at the level of line-core formation, i.e. in the low to mid chromosphere. We also find evidence that the field strength is distributed around these average values with a FWHM that is approximately the same as the average value.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1997A&A...322..985S,

title = {Center-to-limb variation of the second solar spectrum.},

author = {Stenflo, J.O. and Bianda, M. and Keller, C.U. and Solanki, S.K.},

url = {https://www.irsol.usi.ch/data/Papers/AA322_985_1997.pdf},

year  = {1997},

date = {1997-06-01},

urldate = {1997-06-01},

journal = {Astronomy and Astrophysics},

volume = {322},

pages = {985-994},

abstract = {The linear polarization that is caused by scattering processes in the solar atmosphere has been refered to as the "second solar spectrum", since it is structurally as rich as the ordinary intensity spectrum but quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the SrI 4607Å line, which has served as a diagnostic tool for determinations of spatially unresolved, turbulent magnetic fields via the Hanle effect. Here we present the detailed center-to-limb variation of the scattering polarization in this line for a number of new data sets obtained both with an electrooptical modulation system (ZIMPOL) and a non-modulating beam splitter system (at IRSOL, Locarno), to provide improved observational constraints for theoretical modelling. The amplitude and width of the polarization profile, the amount of continuum polarization, as well as the depth and width of the intensity profile have been evaluated and carefully corrected for spectral broadening and stray light. While there is generally good agreement between the five data sets, some systematic differences are shown to be of solar rather than instrumental origin, most likely due to spatially varying Hanle depolarization across the solar disk. A number of other spectral lines have been observed with the ZIMPOL system at two different limb distances (μ=0.1 and 0.2) to allow us to compare the steepness of the center-to-limb variation of their polarization amplitudes. The steepest variation is exhibited by the continuum polarization, which declines by approximately a factor of 6 when going the 15 arcsec distance from μ=0.1 to μ=0.2. The spectral lines with the steepest center-to-limb variation are molecular lines, the CaII infrared triplet, and Hα. In contrast the SrI 4607 and BaII 4554Å lines have only moderately steeper center-to-limb variations than that of an ideal, purely dipole-scattering atmosphere, for which the polarization ratio between μ=0.1 and μ=0.2 is 1.38. These center-to-limb variations may be used to constrain temperature-density models of the upper photosphere and chromosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1997A&A...321..921S,

title = {Problems in measuring prominence oscillations.},

author = {Sütterlin, P. and Wiehr, E. and Bianda, M. and Kueveler, G.},

url = {https://www.irsol.usi.ch/data/Papers/AA321-921.pdf},

year  = {1997},

date = {1997-05-01},

urldate = {1997-05-01},

journal = {Astronomy and Astrophysics},

volume = {321},

pages = {921-926},

abstract = {Time variations of Doppler shifts of the Ca^+^ 8542Å emission in quiescent solar prominences have been measured. A new type of 'limb guider' assures a highly constant distance of the spectrograph slit from the solar limb and furthermore removes low frequency image motion in the direction perpendicular to the slit. Remaining image motion along the slit is usually removed by a correlation of subsequent spectra. This procedure, however, cannot be applied globally to the whole spatial height in the spectra if individual structures exhibit lateral motions along the slit or even decay or arise during the observation. We therefore correlate defined individual emission maxima from successive spectra. The finally obtained power spectra show oscillations with a variety of periods at restricted locations. The data favour the known general presence of periods near 20 and 60-min, however they give only slight indication for `typical' periods near 3 and 5 min.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1996A&A...308..623S,

title = {On the detection of shocks in the solar granulation.},

author = {Solanki, S.K. and Rueedi, I. and Bianda, M. and Steffen, M.},

url = {https://www.irsol.usi.ch/data/Papers/1996A+A...308..623S.pdf},

year  = {1996},

date = {1996-01-01},

urldate = {1996-01-01},

journal = {Astronomy and Astrophysics},

volume = {308},

pages = {623-630},

abstract = {We investigate the spectral signature of a hydrodynamic simulation of solar granulation and compare it with high resolution observations. The model gives the correct qualitative trend of increasing line width with decreasing continuum intensity seen by Nesis et al. (1992) and interpreted by them as a sign of post-shock turbulence. We find, however, that the profiles in the dark downflow lanes are broader even when there is no horizontal transonic flow or shock in the vicinity. We conclude that the observations of Nesis et al. do not provide any firm evidence for the presence of granular transonic flows. Nevertheless, the simulation predicts a promising diagnostic of shocked horizontal flows. We find that at the locations of the shocks the line profiles are particularly broad, especially near the solar limb. We present observations of quiet solar regions that show this specific signature of shocks and thus support the theoretical prediction of transonic granular flows.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1993SoPh..145..205W,

title = {Detection of a significant change in the solar diameter},

author = {Wittmann, A.D. and Alge, E. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/1993SoPh..145..205W.pdf},

doi = {10.1007/BF00627996},

year  = {1993},

date = {1993-05-01},

urldate = {1993-05-01},

journal = {Solar Physics},

volume = {145},

pages = {205},

abstract = {Drift-time measurements of the solar diameter made at Izaña and Locarno in 1990-1992 show a significant increase of the observed angular semidiameter when compared to results obtained at the same two sites in 1981. The observed increase of ≈0.4″ is not due to a systematic (or long-term) variation, but seems to reflect a more complicated behaviour with time: As both series of measurements were made around a maximum of the 11-year cycle, it seems that a variation in phase with solar activity (in the sense of the Secchi-Rosa law) can be ruled out.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{1991SoPh..135..243W,

title = {Recent results on the solar diameter},

author = {Wittmann, A.D. and Alge, E. and Bianda, M.},

url = {https://www.irsol.usi.ch/data/Papers/1991SoPh..135..243W.pdf},

doi = {10.1007/BF00147498},

year  = {1991},

date = {1991-10-01},

urldate = {1991-10-01},

journal = {Solar Physics},

volume = {135},

pages = {243-248},

abstract = {Using optically identical telescopes at different sites, we have measured the solar diameter with a drift-scan technique. In order to investigate the cause of the observed fluctuations, we not only compare observations made simultaneously by different observers at the same telescope, but also observations made simultaneously at two different sites. Our main results are: (a) The mean error of a single drift time measurement is ±0.08s(or ± 1.1″) at Izaña and ±0.11 s (or ± 1.7″) at Locarno; this closely corresponds to the angular resolution at those two sites under normal seeing conditions, (b) We find no correlation between observations at different sites; a significant correlation exists, however, between observations made simultaneously by different observers at the same site: This indicates that most of the observed fluctuations are due to atmospheric effects (`image motion') rather than personality effects, (c) The mean solar semi-diameter derived from a total of 1122 observations made in 1990 (472 at Izaña, 650 at Locarno) is R = (960.56 ± 0.03)″ (Izaña: 960.51″, Locarno: 960.59″); this may be compared with R = (960.32 ± 0.02)″ which is obtained from a re-analysis of 1773 observations made in 1981 (Izaña: 960.16″, Locarno: 960.38″). Although a small residual increase of the solar diameter during the last ten years seems to be indicated, we conclude that most - if not all - of the observed variations are due to variable seeing conditions, and that there is still no conclusive evidence for a genuine solar variation with amplitudes in excess of about ±0.3″. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}