Publications

@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{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{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}
}

@article{NDACYAYISENGA2020c,
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}
}

@article{Upadhyay_2019,
title = {Solar Radio Observation Using CALLISTO at the USO/PRL, Udaipur},
author = {Kushagra Upadhyay and Bhuwan Joshi and Prabir K Mitra and R Bhattacharyya and Divya Oberoi and Christian Monstein},
url = {http://dx.doi.org/10.1109/IMaRC45935.2019.9118669
https://arxiv.org/pdf/2007.01655.pdf},
doi = {10.1109/imarc45935.2019.9118669},
isbn = {9781728140407},
year = {2019},
date = {2019-12-01},
journal = {2019 IEEE MTT-S International Microwave and RF Conference (IMARC)},
publisher = {IEEE},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2019ApJ...883..188S,
title = {Polarized Line Formation in Arbitrary Strength Magnetic Fields: The Case of a Two-level Atom with Hyperfine Structure Splitting},
author = {M. {Sampoorna} and K.N. {Nagendra} and K. {Sowmya} and J. O. {Stenflo} and L.S. {Anusha}},
doi = {10.3847/1538-4357/ab3805},
year = {2019},
date = {2019-09-01},
journal = {Astrophysical Journal},
volume = {883},
number = {2},
pages = {188},
abstract = {Quantum interference effects, together with partial frequency
redistribution (PFR) in line scattering, produce subtle
signatures in the so-called Second Solar Spectrum (the linearly
polarized spectrum of the Sun). These signatures are modified in
the presence of arbitrary strength magnetic fields via the
Hanle, Zeeman, and Paschen─Back effects. In the present paper we
solve the problem of polarized line formation in a magnetized
atmosphere taking into account scattering in a two-level atom
with hyperfine structure splitting together with PFR. To this
end we incorporate the collisionless PFR matrix derived in
Sowmya et al. in the polarized transfer equation. We apply the
scattering expansion method to solve this transfer equation. We
study the combined effects of PFR and the Paschen─Back effect on
polarized line profiles formed in an isothermal one-dimensional
planar atmosphere. For this purpose, we consider the cases of
D$_2$ lines of Li I and Na I.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{2019Ap&SS.364..143S,
title = {Origin of the cosmological constant},
author = {J. O. {Stenflo}},
doi = {10.1007/s10509-019-3636-7},
year = {2019},
date = {2019-09-01},
journal = {Astrophysics and Space Science},
volume = {364},
number = {9},
pages = {143},
abstract = {The observed value of the cosmological constant corresponds to a time
scale that is very close to the current conformal age of the
universe. Here we show that this is not a coincidence but is
caused by a periodic boundary condition, which only manifests
itself when the metric is represented in Euclidian spacetime.
The circular property of the metric in Euclidian spacetime
becomes an exponential evolution (de Sitter or
ensuremathŁambda term) in ordinary spacetime. The value of
ensuremathŁambda then gets uniquely linked to the period
in Euclidian conformal time, which corresponds to the conformal
age of the universe. Without the use of any free model
parameters we predict the value of the dimensionless parameter
ensuremathØmega_ensuremathŁambda to be 67.2%,
which is within 2ensuremathsigma of the value derived from
CMB observations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}