Publications

@article{2021RSPTA.37900177V,
title = {On the influence of magnetic topology on the propagation of internal gravity waves in the solar atmosphere},
author = {G {Vigeesh} and M {Roth} and O {Steiner} and B {Fleck}},
url = {https://arxiv.org/abs/2010.06926},
doi = {10.1098/rsta.2020.0177},
year = {2021},
date = {2021-02-01},
journal = {Philosophical Transactions of the Royal Society of London Series A},
volume = {379},
number = {2190},
pages = {20200177},
abstract = {The solar surface is a continuous source of internal gravity waves
(IGWs). IGWs are believed to supply the bulk of the wave energy
for the lower solar atmosphere, but their existence and role for
the energy balance of the upper layers is still unclear, largely
due to the lack of knowledge about the influence of the Sun's
magnetic fields on their propagation. In this work, we look at
naturally excited IGWs in realistic models of the solar
atmosphere and study the effect of different magnetic field
topographies on their propagation. We carry out radiation-
magnetohydrodynamic simulations of a magnetic field free and two
magnetic models-one with an initial, homogeneous, vertical field
of 100 G magnetic flux density and one with an initial
horizontal field of 100 G flux density. The propagation
properties of IGWs are studied by examining the phase-difference
and coherence spectra in the k$_ħ$ - ensuremathømega
diagnostic diagram. We find that IGWs in the upper solar
atmosphere show upward propagation in the model with
predominantly horizontal field similar to the model without
magnetic field. In contrast to that the model with predominantly
vertical fields show downward propagation. This crucial
difference in the propagation direction is also revealed in the
difference in energy transported by waves for heights below 0.8
Mm. Higher up, the propagation properties show a peculiar
behaviour, which require further study. Our analysis suggests
that IGWs may play a significant role in the heating of the
chromospheric layers of the internetwork region where horizontal
fields are thought to be prevalent.
textbackslashtextbackslashThis article is part of the Theo
Murphy meeting issue `High-resolution wave dynamics in the lower
solar atmosphere'.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2021RSPTA.37900170F,
title = {Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere},
author = {B {Fleck} and M {Carlsson} and E {Khomenko} and M {Rempel} and O {Steiner} and G {Vigeesh}},
url = {https://arxiv.org/abs/2007.05847},
doi = {10.1098/rsta.2020.0170},
year = {2021},
date = {2021-02-01},
journal = {Philosophical Transactions of the Royal Society of London Series A},
volume = {379},
number = {2190},
pages = {20200170},
abstract = {There has been tremendous progress in the degree of realism of three-
dimensional radiation magneto-hydrodynamic simulations of the
solar atmosphere in the past decades. Four of the most
frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D
and MURaM. Here we test and compare the wave propagation
characteristics in model runs from these four codes by measuring
the dispersion relation of acoustic-gravity waves at various
heights. We find considerable differences between the various
models. The height dependence of wave power, in particular of
high-frequency waves, varies by up to two orders of magnitude
between the models, and the phase difference spectra of several
models show unexpected features, including
ensuremathpm180textdegree phase jumps.
textbackslashtextbackslashThis article is part of the Theo
Murphy meeting issue `High-resolution wave dynamics in the lower
solar atmosphere'.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2021RSPTA.37900182K,
title = {On the effect of oscillatory phenomena on Stokes inversion results},
author = {P H {Keys} and O {Steiner} and G {Vigeesh}},
url = {https://arxiv.org/abs/2008.05539},
doi = {10.1098/rsta.2020.0182},
year = {2021},
date = {2021-02-01},
journal = {Philosophical Transactions of the Royal Society of London Series A},
volume = {379},
number = {2190},
pages = {20200182},
abstract = {Stokes inversion codes are crucial in returning properties of the solar
atmosphere, such as temperature and magnetic field strength.
However, the success of such algorithms to return reliable
values can be hindered by the presence of oscillatory phenomena
within magnetic wave guides. Returning accurate parameters is
crucial to both magnetohydrodynamics (MHD) studies and solar
physics in general. Here, we employ a simulation featuring
propagating MHD waves within a flux tube with a known driver and
atmospheric parameters. We invert the Stokes profiles for the
6301 Å and 6302 Å line pair emergent from the
simulations using the well-known Stokes Inversions from Response
functions code to see if the atmospheric parameters can be
returned for typical spatial resolutions at ground-based
observatories. The inversions return synthetic spectra
comparable to the original input spectra, even with asymmetries
introduced in the spectra from wave propagation in the
atmosphere. The output models from the inversions match closely
to the simulations in temperature, line-of-sight magnetic field
and line-of-sight velocity within typical formation heights of
the inverted lines. Deviations from the simulations are seen
away from these height regions. The inversions results are less
accurate during passage of the waves within the line formation
region. The original wave period could be recovered from the
atmosphere output by the inversions, with empirical mode
decomposition performing better than the wavelet approach in
this task. textbackslashtextbackslashThis article is part of
the Theo Murphy meeting issue `High-resolution wave dynamics in
the lower solar atmosphere'.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2021A&A...645A...4P,
title = {Fast and accurate approximation of the angle-averaged redistribution function for polarized radiation},
author = {A {Paganini} and B {Hashemi} and E {Alsina Ballester} and L {Belluzzi}},
url = {https://arxiv.org/abs/2010.03508},
doi = {10.1051/0004-6361/201937149},
year = {2021},
date = {2021-01-01},
journal = {Astronomy and Astrophysics},
volume = {645},
pages = {A4},
abstract = {Context. Modeling spectral line profiles taking frequency redistribution
effects into account is a notoriously challenging problem from
the computational point of view, especially when polarization
phenomena (atomic polarization and polarized radiation) are
taken into account. Frequency redistribution effects are
conveniently described through the redistribution function
formalism, and the angle-averaged approximation is often
introduced to simplify the problem. Even in this case, the
evaluation of the emission coefficient for polarized radiation
remains computationally costly, especially when magnetic fields
are present or complex atomic models are considered.
textbackslash Aims: We aim to develop an efficient algorithm to
numerically evaluate the angle-averaged redistribution function
for polarized radiation. textbackslash Methods: The proposed
approach is based on a low-rank approximation via trivariate
polynomials whose univariate components are represented in the
Chebyshev basis. textbackslash Results: The resulting algorithm
is significantly faster than standard quadrature-based schemes
for any target accuracy in the range [10$^-6$, 10$^-2$].},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{NDACYAYISENGA2020d,
title = {Solar activity, Solar Radio Type III Bursts, e-CALLISTO, Space weather},
author = {Theogene Ndacyayisenga and Jean Uwamahoro and Sasikumar K Raja and Christian Monstein},
url = {http://www.sciencedirect.com/science/article/pii/S027311772030822X},
doi = {10.1016/j.asr.2020.11.022},
issn = {0273-1177},
year = {2020},
date = {2020-12-08},
journal = {Advances in Space Research},
number = {0273-1177},
abstract = {Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{refId0c,
title = {Imaging spectropolarimetry for magnetic field diagnostics in solar prominences},
author = {R {Di Campli} and R Ramelli and M Bianda and I Furno and S Kumar Dhara and L Belluzzi},
url = {https://doi.org/10.1051/0004-6361/202037931},
doi = {10.1051/0004-6361/202037931},
year = {2020},
date = {2020-12-01},
journal = {A&A},
volume = {644},
pages = {A89},
abstract = {Context. Narrowband imaging spectropolarimetry is one of the most powerful tools available to infer information about the intensity and topology of the magnetic fields present in extended plasma structures in the solar atmosphere.

Aims. We describe the instrumental set-up and the observing procedure that we have developed and optimized at the Istituto Ricerche Solari Locarno in order to perform imaging spectropolarimetry. A measurement that highlights the potential of the ensuing observations for magnetic field diagnostics in solar prominences is presented.

Methods. Monochromatic images of solar prominences were obtained by combining a tunable narrowband filter, based on two Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and circular polarization were measured at every pixel of the monochromatic image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was performed across the profile of the considered spectral line. The HAZEL inversion code was applied to the observed Stokes profiles to infer a series of physical properties of the observed structure.

Results. We carried out a spectropolarimetric observation of a prominence, consisting of a set of quasi-monochromatic images across the He I D3 line at 5876 Å in the four Stokes parameters. The map of observed Stokes profiles was inverted with HAZEL, finding magnetic fields with intensities between 15 and 30 G and directed along the spine of the prominence, which is in agreement with the results of previous works.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Mahender2020,
title = {A Statistical Study of Low-Frequency Solar Radio Type III Bursts},
author = {Aroori Mahender and K {Sasikumar Raja} and R Ramesh and Vemareddy Panditi and Christian Monstein and Yellaiah Ganji},
doi = {10.1007/s11207-020-01722-z},
issn = {1573-093X},
year = {2020},
date = {2020-11-09},
journal = {Solar Physics},
volume = {295},
number = {11},
pages = {153},
abstract = {We have studied low-frequency (45 – 410 MHz) type III solar radio bursts observed using the e-Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) spectrometer located at Gauribidanur Radio Observatory, India, during 2013 – 2017. After inspecting 1531 type III bursts we found that 426 bursts were associated with flares, while the others might have been triggered by small scale features/weak energy events present in the solar corona. In this study, we have carried out a statistical analysis of various observational parameters like start time, lower- and upper-frequency cut-offs of type III bursts and their association with flares, variation of such parameters with flare parameters such as location, class, onset, and peak times. From this study, we found that most of the high frequency bursts (whose upper-frequency cut-off is >350MHz$>350~mboxMHz$) originate from western longitudes. We interpret that this could be due to the fact that Parker spirals from these longitudes are directed towards the Earth and high frequency bursts are more directive. Further we report that the number of bursts that reach Earth from western longitudes is higher than from eastern longitudes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2020ApJ...903L..10F,
title = {Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale},
author = {C ~E {Fischer} and G {Vigeesh} and P {Lindner} and J ~M {Borrero} and F {Calvo} and O {Steiner}},
url = {https://arxiv.org/abs/2010.05577},
doi = {10.3847/2041-8213/abbada},
year = {2020},
date = {2020-11-01},
journal = {Astrophysical Journal, Letters},
volume = {903},
number = {1},
pages = {L10},
abstract = {Using high-resolution spectropolarimetric data recorded with the Swedish
1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be
the observational signature of underlying tubes of vortical flow
with their axis oriented parallel to the solar surface.
Associated with these horizontal vortex tubes, we detect in some
cases a significant signal in linear polarization, located at
the trailing dark edge of the granular lane. The linear
polarization appears at a later stage of the granular lane
development, and is flanked by patches of circular polarization.
Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field
aligned with the granular lane. We analyze snapshots of a
magnetohydrodynamic numerical simulation and find cases in which
the horizontal vortex tube of the granular lane redistributes
and transports the magnetic field to the solar surface causing a
polarimetric signature similar to what is observed. We thus
witness a mechanism capable of transporting magnetic flux to the
solar surface within granules. This mechanism is probably an
important component of the small-scale dynamo supposedly acting
at the solar surface and generating the quiet-Sun magnetic
field.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2020JPhCo...4j5001S,
title = {Cosmological constant caused by observer-induced boundary condition},
author = {Jan O {Stenflo}},
url = {https://arxiv.org/abs/2010.07743},
doi = {10.1088/2399-6528/abbab8},
year = {2020},
date = {2020-10-01},
journal = {Journal of Physics Communications},
volume = {4},
number = {10},
pages = {105001},
abstract = {The evolution of the wave function in quantum mechanics is deterministic
like that of classical waves. Only when we bring in observers
the fundamentally different quantum reality emerges. Similarly
the introduction of observers changes the nature of spacetime by
causing a split between past and future, concepts that are not
well defined in the observer-free world. The induced temporal
boundary leads to a resonance condition for the oscillatory
vacuum solutions of the metric in Euclidean time. It corresponds
to an exponential de Sitter evolution in real time, which can be represented by a cosmological constant $rmŁambda =2pi
^2/r_u^2$ , where r$_u$ is the radius of the particle
horizon at the epoch when the observer exists. For the present
epoch we get a value of ensuremathŁambda that agrees with
the observed value within 2ensuremathsigma of the
observational errors. This explanation resolves the cosmic
coincidence problem. Our epoch in cosmic history does not herald
the onset of an inflationary phase driven by some dark energy.
We show that the observed accelerated expansion that is deduced
from the redshifts is an édge effect' due to the observer-
induced boundary and not representative of the intrinsic
evolution. The new theory satisfies the BBN (Big Bang
nucleosynthesis) and CMB (cosmic microwave background)
observational constraints equally well as the concordance model
of standard cosmology. There is no link between the dark energy
and dark matter problems. Previous conclusions that dark matter
is mainly non-baryonic are not affected.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{10.1093/mnras/staa2638,
title = {Towards a polarization prediction for LISA via intensity interferometry},
author = { Sandra Baumgartner and Mauro Bernardini and José R Canivete Cuissa and Hugues de Laroussilhe and Alison M W Mitchell and Benno A Neuenschwander and Prasenjit Saha and Timothée Schaeffer and Deniz Soyuer and Lorenz Zwick},
url = {https://doi.org/10.1093/mnras/staa2638},
doi = {10.1093/mnras/staa2638},
issn = {0035-8711},
year = {2020},
date = {2020-09-02},
journal = {Monthly Notices of the Royal Astronomical Society},
volume = {498},
number = {3},
pages = {4577-4589},
abstract = {Compact Galactic Binary Systems with orbital periods of a few hours are expected to be detected in gravitational waves (GW) by Laser Interferometer Space Antenna (LISA) or a similar mission. At present, these so-called verification binaries provide predictions for GW frequency and amplitude. A full polarization prediction would provide a new method to calibrate LISA and other GW observatories, but requires resolving the orientation of the binary on the sky, which is not currently possible. We suggest a method to determine the elusive binary orientation and hence predict the GW polarization, using km-scale optical intensity interferometry. The most promising candidate is CD–30° 11223, consisting of a hot helium subdwarf with mB = 12 and a much fainter white dwarf companion, in a nearly edge-on orbit with period 70.5 min. We estimate that the brighter star is tidally stretched by 6 per cent. Resolving the tidal stretching would provide the binary orientation. The resolution needed is far beyond any current instrument, but not beyond current technology. We consider scenarios where an array of telescopes with km-scale baselines and/or the Very Large Telescope (VLT) and Extremely Large Telescope (ELT) are equipped with recently developed kilopixel sub-ns single-photon counters and used for intensity interferometry. We estimate that a team-up of the VLT and ELT could measure the orientation to ±1° at 2σ confidence in 24 h of observation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2020A&A...641A..63C,
title = {Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line},
author = { Emilia Capozzi and Ernest Alsina Ballester and Luca Belluzzi and Michele Bianda and Sajal Kumar Dhara and Renzo Ramelli},
url = {https://arxiv.org/abs/2006.13653},
doi = {10.1051/0004-6361/202038455},
year = {2020},
date = {2020-09-01},
journal = {Astronomy and Astrophysics},
volume = {641},
pages = {A63},
abstract = {Context. Several strong resonance lines, such as H I
Ly-ensuremathalpha, Mg II k, Ca II K, and Ca I 4227
Å, are characterized by deep and broad absorption profiles
in the solar intensity spectrum. These resonance lines show
conspicuous linear scattering polarization signals when observed
in quiet regions close to the solar limb. Such signals show a
characteristic triplet-peak structure with a sharp peak in the
line core and extended wing lobes. The line core peak is
sensitive to the presence of magnetic fields through the Hanle
effect, which however is known not to operate in the line wings.
Recent theoretical studies indicate that, contrary to what was
previously believed, the wing linear polarization signals are
also sensitive to the magnetic field through magneto-optical
(MO) effects. textbackslash Aims: We search for observational
indications of this recently discovered physical mechanism in
the scattering polarization wings of the Ca I 4227 Å line.
textbackslash Methods: We performed a series of
spectropolarimetric observations of this line using the Zurich
IMaging POLarimeter camera at the Gregory-Coudé telescope at
Istituto Ricerche Solari Locarno in Switzerland and at the
GREGOR telescope in Tenerife (Spain). textbackslash Results:
Spatial variations of the total linear polarization degree and
linear polarization angle are clearly appreciable in the wings
of the observed line. We provide a detailed discussion of our
observational results, showing that the detected variations
always take place in regions in which longitudinal magnetic
fields are present, thus supporting the theoretical prediction
that they are produced by MO effects.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{refId0b,
title = {Vortices evolution in the solar atmosphere - A dynamical equation for the swirling strength},
author = {José R. Canivete Cuissa, Oskar Steiner},
url = {https://doi.org/10.1051/0004-6361/202038060},
doi = {10.1051/0004-6361/202038060},
year = {2020},
date = {2020-07-21},
journal = {Astronomy and Astrophysics},
volume = {639},
pages = {A118},
abstract = {Aims. We study vortex dynamics in the solar atmosphere by employing and deriving the analytical evolution equations of two vortex identification criteria.

Methods. The two criteria used are vorticity and the swirling strength. Vorticity can be biased in the presence of shear flows, but its dynamical equation is well known; the swirling strength is a more precise criterion for the identification of vortical flows, but its evolution equation is not known yet. Therefore, we explore the possibility of deriving a dynamical equation for the swirling strength. We then apply the two equations to analyze radiative magneto-hydrodynamical simulations of the solar atmosphere produced with the CO5BOLD code.

Results. We present a detailed review of the swirling strength criterion and the mathematical derivation of its evolution equation. This equation did not exist in the literature before and it constitutes a novel tool that is suitable for the analysis of a wide range of problems in (magneto-)hydrodynamics. By applying this equation to numerical models, we find that hydrodynamical and magnetic baroclinicities are the driving physical processes responsible for vortex generation in the convection zone and the photosphere. Higher up in the chromosphere, the magnetic terms alone dominate. Moreover, we find that the swirling strength is produced at small scales in a chaotic fashion, especially inside magnetic flux concentrations.

Conclusions. The swirling strength represents an appropriate criterion for the identification of vortices in turbulent flows, such as those in the solar atmosphere. Moreover, its evolution equation, which is derived in this paper, is pivotal for obtaining precise information about the dynamics of these vortices and the physical mechanisms responsible for their production and evolution. Since this equation is available, the swirling strength is now the ideal quantity to study the dynamics of vortices in (magneto-)hydrodynamics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2020ApJ...898...49N,
title = {Importance of Angle-dependent Partial Frequency Redistribution in Hyperfine Structure Transitions Under the Incomplete Paschen-Back Effect Regime},
author = {K N {Nagendra} and K {Sowmya} and M {Sampoorna} and J O {Stenflo} and L S {Anusha}},
url = {https://arxiv.org/abs/2007.04044},
doi = {10.3847/1538-4357/ab9747},
year = {2020},
date = {2020-07-01},
journal = {Astrophysical Journal},
volume = {898},
number = {1},
pages = {49},
abstract = {Angle-frequency coupling in scattering of polarized light on atoms is
represented by the angle-dependent (AD) partial frequency
redistribution (PRD) matrices. There are several lines in the
linearly polarized solar spectrum, for which PRD combined with
quantum interference between hyperfine structure states play a
significant role. Here we present the solution of the polarized
line transfer equation including the AD-PRD matrix for
scattering on a two-level atom with hyperfine structure
splitting and an unpolarized lower level. We account for the
effects of arbitrary magnetic fields (including the incomplete
Paschen-Back effect regime) and elastic collisions. For
exploratory purposes we consider a self-emitting isothermal
planar atmosphere and use atomic parameters that represent an
isolated Na I D$_2$ line. For this case we show that the AD-
PRD effects are significant for field strengths below about 30
G, but that the computationally much less demanding
approximation of angle-averaged PRD may be used for stronger
fields.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{WanMokhtar2020,
title = {Data background levels of the metre and decimetre wavelength observations by E-CALLISTO network: the Gauribidanur and Greenland sites},
author = {W Z A {Wan Mokhtar} and Z S Hamidi and Z Z Abidin and Z A Ibrahim and C Monstein},
doi = {10.1007/s12648-020-01765-9},
issn = {0974-9845},
year = {2020},
date = {2020-06-22},
journal = {Indian Journal of Physics},
abstract = {The instability of the Sun’s magnetic field can ignite many eruptive events on the solar surface, including flares, coronal mass ejections, and prominence eruptions. The inner heliosphere environment is affected, and consequently, these events are said to contribute to the celestial weather change. As one of the many eruptive events, a solar flare is of the most frequent due to the magnetic reconnection process in which the accelerated electrons from the reconnection sites escape into the interplanetary space and cause solar radio bursts type III (SRBT III). When it is observed near the Earth, this SRBT III is in the form of radio dynamics spectrum; thus, monitoring this spectrum is vital to the further analysis of the said SRBT III. In this paper, we investigate the background levels: short and long periods of the CALLISTO instruments from two different stations where for each site, a 10-day background-level observation is randomly selected. For the purpose of this study, the mean differences and coefficient of variation (CV) distributions for every frequency channel are determined where most of the frequency channels have displayed small mean differences between these two background levels: short and long periods and the CV distributions as well. These short-period observations, within 15 min of the background levels, are found significant enough to warrant further analysis of the solar radio bursts detected by the CALLISTO instruments.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2020ApJ...893L..44Z,
title = {Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line},
author = {Franziska {Zeuner} and Rafael {Manso Sainz} and Alex {Feller} and Michiel {van Noort} and Sami K {Solanki} and Francisco A {Iglesias} and Kevin {Reardon} and Valentin {Martinez Pillet}},
doi = {10.3847/2041-8213/ab86b8},
year = {2020},
date = {2020-04-01},
journal = {Astrophysical Journal, Letters},
volume = {893},
number = {2},
pages = {L44},
abstract = {Magnetic fields in turbulent, convective high-ensuremathbeta
plasma naturally develop highly tangled and complex topologies -
the solar photosphere being the paradigmatic example. These
fields are mostly undetectable by standard diagnostic techniques
with finite spatio-temporal resolution due to cancellations of
Zeeman polarization signals. Observations of resonance
scattering polarization have been considered to overcome these
problems. But up to now, observations of scattering polarization
lack the necessary combination of high sensitivity and high
spatial resolution in order to directly infer the turbulent
magnetic structure at the resolution limit of solar telescopes.
Here, we report the detection of clear spatial structuring of
scattering polarization in a magnetically quiet solar region at
disk center in the Sr I 4607 Å spectral line on granular
scales, confirming theoretical expectations. We find that the
linear polarization presents a strong spatial correlation with
the local quadrupole of the radiation field. The result
indicates that polarization survives the dynamic and turbulent
magnetic environment of the middle photosphere and is thereby
usable for spatially resolved Hanle observations. This is an
important step toward the long-sought goal of directly observing
turbulent solar magnetic fields at the resolution limit and
investigating their spatial structure.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Pauzi2020,
title = {Investigation into CME Shock Speed Resulting from Type II Solar Radio Bursts},
author = {F A M Pauzi and Z Z Abidin and S J Guo and G N Gao and L Dong and C Monstein},
doi = {10.1007/s11207-019-1404-z},
issn = {1573-093X},
year = {2020},
date = {2020-03-18},
journal = {Solar Physics},
volume = {295},
number = {3},
pages = {42},
abstract = {An investigation into Type II solar radio bursts was carried out to understand the frequency gap between fundamental and harmonic emissions of the radio burst. This investigation focused on Type II solar radio bursts with flares and coronal mass ejections by relating the separation between fundamental and harmonic emissions. We used the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Spectrometers (CALLISTO) and a newly designed low-frequency antenna array. This article describes the proposed new instrument in terms of its antenna design, the bandpass testing of the antenna, the new system significance in studying Type II solar radio bursts, and its comparison with other leading radio solar monitoring instruments. Upon setting up the new technology, the radio-frequency interference of the observation site at the University of Malaya was shown to emphasize the suitability of the selected site. This article also shows the preliminary results of the proposed new instrument by reporting the detection of a Type III solar radio burst that was confirmed by CALLISTO. Moreover, it also includes the optimal observation design and strategies for future detections.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Prieto2020,
title = {Increase in Interference Levels in the 45 – 870 MHz Band at the Spanish e-CALLISTO Sites over the Years 2012 and 2019},
author = {Manuel Prieto and Javier Bussons Gordo and Javier Rodríguez-Pacheco and Agustín Martínez and Sebastián Sánchez and Andrés Russu and Christian Monstein and Rafael Fernández},
doi = {10.1007/s11207-019-1577-5},
issn = {1573-093X},
year = {2020},
date = {2020-01-15},
journal = {Solar Physics},
volume = {295},
number = {2},
pages = {11},
abstract = {Two sets of radio-frequency interference (RFI) measurements in the 45 – 870 MHz band are compared. The first set was taken in 2012 at various sites in the province of Guadalajara (Spain) as part of a worldwide site-testing campaign for the deployment of an international network of solar radio-spectrometers, the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) array. Peralejos de las Truchas was found to be an ideal location, even for high-sensitivity non-solar observations, with the lowest interference levels ever measured in the framework of e-CALLISTO. The same set of measurements have been repeated seven years later using the same experimental setup at the same locations. The results presented in this article show that the RFI levels after seven years have notably increased, at some places by a factor of two, thereby placing at risk broadband spectroscopic radio-astronomy studies from the ground.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{NDACYAYISENGA2020,
author = {Theogene Ndacyayisenga and Jean Uwamahoro and Sasikumar K Raja and Christian Monstein},
url = {http://www.sciencedirect.com/science/article/pii/S027311772030822X},
doi = {https://doi.org/10.1016/j.asr.2020.11.022},
issn = {0273-1177},
year = {2020},
date = {2020-01-01},
abstract = {Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{NDACYAYISENGA2020b,
author = {Theogene Ndacyayisenga and Jean Uwamahoro and Sasikumar K Raja and Christian Monstein},
url = {http://www.sciencedirect.com/science/article/pii/S027311772030822X},
doi = {10.1016/j.asr.2020.11.022},
issn = {0273-1177},
year = {2020},
date = {2020-01-01},
abstract = {Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}