milic

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Author name code: milic
ADS astronomy entries on 2022-09-14
author:"Milic, Ivan"

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Title: Investigating magnetic field inference from the spectral region
around the Mg I b2 line using the weak-field approximation
Authors: Vukadinović, D.; Milić, I.; Atanacković, O.
2022A&A...664A.182V Altcode: 2022arXiv220504236V
Context. The understanding of the magnetic field structure in the
solar atmosphere is important in assessing both the dynamics and
the energy balance of the solar atmosphere. Our knowledge about
these magnetic fields comes predominantly from the interpretation
of spectropolarimetric observations. Simpler approaches based on
approximations such as the weak-field approximation (WFA) deserve
special attention as key methods in the interpretation of large,
high-resolution datasets. Aims: We investigate the applicability
of the WFA for retrieving the depth-dependent line-of-sight
(LOS) magnetic field from the spectral region containing the Mg I
b2 spectral line and two photospheric Ti I and Fe I lines in
its wings. Methods: We constructed and applied a 12-level model
for Mg I atom that realistically reproduces the b2 line
profile of the mean quiet Sun. We tested the applicability of the WFA
to the spectra computed from the FAL C atmospheric model with different
magnetic and velocity fields added on an ad hoc basis . Then we extended
the analysis to the spectra computed from two 3D magneto-hydrodynamic
(MHD) MURaM simulations of the solar atmosphere. The first MHD cube
was used to estimate the Stokes V formation heights of each spectral
line. These heights correspond to optical depths at which the standard
deviation of the difference between the WFA-inferred magnetic field
and the magnetic field in the MHD cube is minimal. The estimated
formation heights were verified using the second MHD cube. 
Results: The LOS magnetic field retrieved by the WFA is reliable for
the magnetic field strength up to 1.4 kG even when moderate velocity
gradients are present. The exception is the Fe I line, for which we
found a strong discrepancy in the WFA-inferred magnetic fields because
of the line blend. We estimated the Stokes V formation heights of each
spectral line to be: logτFe = −2.6, logτMg
= −3.3, and logτTi = −1.8. We were able to estimate
the LOS magnetic field from the MURaM cube at these heights with the
uncertainty of 150 G for the Fe I and Ti I lines and only 40 G for
the Mg I b2 line. Conclusions: Using the WFA, we can
quickly get a reliable estimate of the structure of the LOS magnetic
field in the observed region. This offers a significant advantage in
comparison with otherwise time-consuming classical spectropolarimetric
inversions. We find that the Mg I b2 line profile calculated
from the quiet Sun MURaM simulation agrees very well with the observed
mean spectrum of the quiet Sun.

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Title: Observational Approach to Computing the Poynting Flux in the
Quiet Sun Photosphere
Authors: Tilipman, Dennis; Martínez Pillet, Valentin; Tremblay,
Benoit; Kazachenko, Maria; Milic, Ivan; Yadav, Rahul
2022cosp...44.2516T Altcode:
Understanding magnetically-driven processes in the quiet Sun is
crucial for understanding chromospheric and coronal heating. The main
goal of our study is to quantify the energy output of the quiet Sun
photosphere. The amount of magnetic energy being transported upwards
from the photosphere can be expressed in terms of the vertical component
of Poynting flux, which is a cross-product of magnetic and electric
fields. While magnetic fields and energy fluxes within active regions
and plage have been evaluated before, quiet Sun magnetograms have only
recently become available with the launch of missions such as Hinode
and Sunrise and the Daniel K. Inouye Solar Telescope (DKIST) coming
online early this year. In this presentation, we present estimates
of Poynting flux using IMaX data. As the electric field E is one
of the two principal quantities required to compute Poynting flux,
we use two distinct approaches to infer E. In the first approach, we
derive the electric field using ideal plasma assumption with horizontal
velocities obtained from the convolutional neural network (DeepVel,
Asensio Ramos et al. 2017). In the second approach, we derive E using
the PDFI-SS approach uncurling Faraday's law (Fisher et al. 2020). We
discuss the distribution of Poynting flux and whether it is sufficient
to explain chromospheric and coronal heating.

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Title: Evaluating Non-LTE Spectral Inversions with ALMA and IBIS
Authors: Hofmann, Ryan A.; Reardon, Kevin P.; Milic, Ivan; Molnar,
Momchil E.; Chai, Yi; Uitenbroek, Han
2022ApJ...933..244H Altcode: 2022arXiv220508760H
We present observations of a solar magnetic network region in the
millimeter continuum with the Atacama Large Millimeter/submillimeter
Array (ALMA) and in the Ca 8542 and Na 5896 Å spectral lines with
the Interferometric Bidimensional Spectrometer (IBIS). Our goal is
to compare the measurement of local gas temperatures provided by ALMA
with the temperature diagnostics provided by non-LTE inversions using
the STockholm inversion Code (STiC). In performing these inversions,
we find that using column mass as the reference height scale, rather
than optical depth, provides more reliable atmospheric profiles above
the temperature minimum and that the treatment of non-LTE hydrogen
ionization brings the inferred chromospheric temperatures into better
agreement with the ALMA measurements. The Band 3 brightness temperatures
are higher but well correlated spatially with the inversion-derived
temperatures at the height of formation of the Ca 8542 line core. The
Band 6 temperatures instead do not show good correlations with the
temperatures at any specific layer in the inverted atmospheres. We then
performed inversions that included the millimeter-continuum intensities
as an additional constraint. Incorporating Band 3 generally resulted in
atmospheres showing a strong temperature rise in the upper atmosphere,
while including Band 6 led to significant regions of anomalously low
temperatures at chromospheric heights. This is consistent with the
idea that the Band 6 emission can come from a combination of heights
ranging from the temperature minimum to upper chromosphere. The
poor constraints on the chromospheric electron density with existing
inversion codes introduces difficulties in determining the height(s)
of formation of the millimeter continuum as well as uncertainties in
the temperatures derived from the spectral lines.

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Title: Quantifying Magnetic Energy Flux in the Quiet Sun Photosphere
using Sunrise/IMaX Observations
Authors: Tilipman, Dennis; Kazachenko, Maria; Tremblay, Benoit;
Martinez-Pillet, Valentin; Milic, Ivan
2021AGUFMSH42B..04T Altcode:
Understanding magnetically-driven processes in the quiet Sun is
crucial for understanding chromospheric and coronal heating. The main
goal of our study is to quantify the energy output of the quiet Sun
photosphere. The amount of magnetic energy can be expressed in terms of
the Poynting flux, which is a cross-product of magnetic and electric
fields. Poynting flux has been computed for active regions and plage,
but the weakness of polarimetric signal in the quiet Sun presents
a bigger challenge. Quiet Sun magnetic fields have only recently
become observable with the launch of missions such as Hinode and
Sunrise. The Daniel K. Inouye Solar Telescope (DKIST) is expected to
further improve the quality of these observations -- both spatial and
temporal resolutions, as well as polarimetric signal, are expected to
improve significantly, allowing us to perform reliable inversions of
magnetic, electric, and velocity fields, all of which are required
to compute the Poynting flux. We test different inversion methods
on Sunrise/IMaX data in order to streamline this process once DKIST
becomes operational and to understand the limitations of these inversion
techniques. In this work, we present our results obtained from velocity
and electric field inversions of photospheric images, magnetograms and
Doppler velocities from Sunrise/IMaX, the challenges associated with
these inversions, and implications for future DKIST observations. We
also discuss approaches to extend this analysis from photosphere to
overlying layers of the atmosphere, which would allow us to study how
these layers respond to magnetic energy injections from below.

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Title: Limitations of the Ca II 8542 Å Line for the Determination
of Magnetic Field Oscillations
Authors: Felipe, Tobias; Socas Navarro, Hector; Sangeetha, C. R.;
Milic, Ivan
2021ApJ...918...47F Altcode: 2021arXiv210702160F
Chromospheric umbral oscillations produce periodic brightenings
in the core of some spectral lines, known as umbral flashes. They
are also accompanied by fluctuations in velocity, temperature, and,
according to several recent works, magnetic field. In this study, we
aim to ascertain the accuracy of the magnetic field determined from
inversions of the Ca II 8542 Å line. We have developed numerical
simulations of wave propagation in a sunspot umbra. Synthetic Stokes
profiles emerging from the simulated atmosphere were computed and
then inverted using the NICOLE code. The atmospheres inferred from
the inversions have been compared with the original parameters from
the simulations. Our results show that the inferred chromospheric
fluctuations in velocity and temperature match the known oscillations
from the numerical simulation. In contrast, the vertical magnetic field
obtained from the inversions exhibits an oscillatory pattern with a ~300
G peak-to-peak amplitude, which is absent in the simulation. We have
assessed the error in the inferred parameters by performing numerous
inversions with slightly different configurations of the same Stokes
profiles. We find that when the atmosphere is approximately at rest,
the inversion tends to favor solutions that underestimate the vertical
magnetic field strength. On the contrary, during umbral flashes,
the values inferred from most of the inversions are concentrated at
stronger fields than those from the simulation. Our analysis provides
a quantification of the errors associated with the inversions of the
Ca II 8542 Å line and suggests caution with the interpretation of
the inferred magnetic field fluctuations.

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Title: The Lightweaver Framework for Nonlocal Thermal Equilibrium
Radiative Transfer in Python
Authors: Osborne, Christopher M. J.; Milić, Ivan
2021ApJ...917...14O Altcode: 2021arXiv210700475O
Tools for computing detailed optically thick spectral line profiles
out of local thermodynamic equilibrium have always been focused on
speed, due to the large computational effort involved. With the
Lightweaver framework, we have produced a more flexible, modular
toolkit for building custom tools in a high-level language, Python,
without sacrificing speed against the current state of the art. The
goal of providing a more flexible method for constructing these complex
simulations is to decrease the barrier to entry and allow more rapid
exploration of the field. In this paper we present an overview of the
theory of optically thick nonlocal thermodynamic equilibrium radiative
transfer, the numerical methods implemented in Lightweaver including
the problems of time-dependent populations and charge-conservation,
as well as an overview of the components most users will interact with,
to demonstrate their flexibility.

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Title: Machine learning initialization to accelerate Stokes profile
inversions
Authors: Gafeira, R.; Orozco Suárez, D.; Milić, I.; Quintero Noda,
C.; Ruiz Cobo, B.; Uitenbroek, H.
2021A&A...651A..31G Altcode: 2021arXiv210309651G
Context. At present, an exponential growth in scientific data
from current and upcoming solar observatories is expected. Most of
the data consist of high spatial and temporal resolution cubes of
Stokes profiles taken in both local thermodynamic equilibrium (LTE)
and non-LTE spectral lines. The analysis of such solar observations
requires complex inversion codes. Hence, it is necessary to develop
new tools to boost the speed and efficiency of inversions and reduce
computation times and costs. Aims: In this work we discuss
the application of convolutional neural networks (CNNs) as a tool to
advantageously initialize Stokes profile inversions. Methods:
To demonstrate the usefulness of CNNs, we concentrate in this paper on
the inversion of LTE Stokes profiles. We use observations taken with
the spectropolarimeter on board the Hinode spacecraft as a test bench
mark. First, we carefully analyse the data with the SIR inversion code
using a given initial atmospheric model. The code provides a set of
atmospheric models that reproduce the observations well. These models
are then used to train a CNN. Afterwards, the same data are again
inverted with SIR but using the trained CNN to provide the initial
guess atmospheric models for SIR. Results: The CNNs allow us
to significantly reduce the number of inversion cycles when used to
compute initial guess model atmospheres (`assisted inversions'),
therefore decreasing the computational time for LTE inversions by
a factor of two to four. CNNs alone are much faster than assisted
inversions, but the latter are more robust and accurate. CNNs also
help to automatically cluster pixels with similar physical properties,
allowing the association with different solar features on the solar
surface, which is useful when inverting huge datasets where completely
different regimes are present. The advantages and limitations of machine
learning techniques for estimating optimum initial atmospheric models
for spectral line inversions are discussed. Finally, we describe a
python wrapper for the SIR and DeSIRe codes that allows for the easy
setup of parallel inversions. The tool implements the assisted inversion
method described in this paper. The parallel wrapper can also be used
to synthesize Stokes profiles with the RH code. Conclusions:
The assisted inversions can speed up the inversion process, but the
efficiency and accuracy of the inversion results depend strongly on
the solar scene and the data used for the CNN training. This method
(assisted inversions) will not obviate the need for analysing individual
events with the utmost care but will provide solar scientists with
a much better opportunity to sample large amounts of inverted data,
which will undoubtedly broaden the physical discovery space.

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Title: Sparse Representation of HINODE/SOT/SP Spectra Using
Convolutional Neural Networks
Authors: Flint, S.; Milic, I.
2021AAS...23821304F Altcode:
A fundamental problem in solar spectropolarimetry is relating observed
spectra and their polarization to the physical parameters of the
underlying atmosphere. One of the difficulties in this process is the
fact that the spectra usually can be represented with a much smaller
number of hyperparameters than what is suggested by the number of
wavelength points used for sampling. Said differently, spectra can
usually be compressed or described in a sparser basis. In this work,
we use the neural networks to investigate the dimensionality of
photospheric spectra, and to compare the compressed spectra with the
maps of physical parameters used to generate the said spectra.

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Title: ALMA and IRIS Observations Highlighting the Dynamics and
Structure of Chromospheric Plage
Authors: Hofmann, R. A.; Reardon, K.; Milic, I.
2021AAS...23820505H Altcode:
Studies of the thermal structure of the solar chromosphere are typically
hampered by the complexities of non-LTE radiative transfer. This issue
can be addressed using observations of the millimeter continuum, which
directly probes the electron temperatures in the chromosphere. In recent
years, the Atacama Large Millimeter/submillimeter Array (ALMA) has made
it possible, for the first time, to obtain millimeter observations of
sufficient spatial resolution to supplement spectral line observations
and inversions. Here, we present observations of a plage in the 3.0
mm and 1.2 mm continua with ~2 arcsecond resolution, combined with
simultaneous imaging spectroscopy observations from the Interferometric
Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. We
compare the observed ALMA brightness temperatures with temperatures
inferred from spectral inversions using the Na D1 5896 Å and Ca II 8542
Å lines, and investigate the wide range of physical heights probed by
the millimeter continuum. We find that the millimeter emission arises
from a range of heights both above and below the chromospheric calcium
line, depending on the local temperature profile and electron densities.

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Title: Are the Magnetic Fields Radial in the Solar Polar Region?
Authors: Sun, Xudong; Liu, Yang; Milić, Ivan; Griñón-Marín,
Ana Belén
2021RNAAS...5..134S Altcode: 2021arXiv210601461S
We investigate the orientation of the photospheric magnetic fields
in the solar polar region using observations from the Helioseismic
and Magnetic Imager (HMI). Inside small patches of significant
polarization, the inferred magnetic field vectors at 1″ scale appear
to systematically deviate from the radial direction. Most tilt toward
the pole; all are more inclined toward the plane of sky compared to the
radial vector. These results, however, depend on the "filling factor"
f that characterizes the unresolved magnetic structures. The default,
uninformative f ≡ 1 for HMI will incur larger inclination and less
radial fields than f &lt; 1. The observed trend may be a systematic
bias inherent to the limited resolution.

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Title: Sparse Representation of HINODE/SOT/SP Spectra Using
Convolutional Neural Networks
Authors: Flint, Serena; Milic, Ivan
2021csss.confE.189F Altcode:
A fundamental problem in solar spectropolarimetry is relating observed
spectra and their polarization to the physical parameters of the
underlying atmosphere. One of the difficulties in this process is the
fact that the spectra usually can be represented with a much smaller
number of hyperparameters than what is suggested by the number of
wavelength points used for sampling. Said differently, spectra can
usually be compressed or described in a sparser basis. In this work,
we use the neural networks to investigate the dimensionality of
photospheric spectra, and to compare the compressed spectra with the
maps of physical parameters used to generate the said spectra.

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Title: Looking for Changes in Photospheric Temperature Gradients
over Solar Cycle 24 Using Hinode/SP
Authors: Crowley, James; Milic, Ivan
2021csss.confE.206C Altcode:
&lt;strong&gt;The intent of this project is to study the effects of the
solar magnetic cycle on the thermal structure of the solar atmosphere
in the quiet Sun. Using data from the spectropolarimeter onboard
the Hinode satellite, four datasets were selected from throughout
Solar Cycle 24; all datasets selected were near the disk center and
without any obvious magnetic signatures. Using an inversion based on
the Milne-Eddington model, a quarter-million pixels were inverted from
each dataset, using two different inversion schemes. By inverting the
data and analyzing the differences in the inverted parameters between
the datasets, we attempt to see if the resolution of the Hinode data
combined with a Milne-Eddington approach is able to detect meaningful
differences in photospheric structure throughout the solar cycle,
primarily the source function and its gradient. Our results so far
suggest that more detailed inversion and /or data preprocessing is
needed to detect eventual presence of the changes. &lt;/strong&gt;

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Title: ALMA observations and spectral inversions - what can we learn
about the Sun and our techniques?
Authors: Hofmann, R.; Reardon, K.; Milic, I.
2020AGUFMSH0010002H Altcode:
Studies of the thermal structure of the solar chromosphere are typically
hampered by the complexities of non-LTE radiative transfer. This issue
can be addressed using observations of the millimeter continuum, which
directly probes the electron temperatures in the chromosphere. In recent
years, the Atacama Large Millimeter/submillimeter Array (ALMA) has made
it possible, for the first time, to obtain millimeter observations of
sufficient spatial resolution to supplement spectral line observations
and inversions. Here, we present observations of a plage in the 3.0
mm and 1.2 mm continua with ~2 arcsecond resolution, combined with
simultaneous imaging spectroscopy observations from the Interferometric
Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. We
compare the observed ALMA brightness temperatures with temperatures
inferred from spectral inversions using the Na D1 5896 Å and Ca II 8542
Å lines, and investigate the wide range of physical heights probed by
the millimeter continuum. We find that the millimeter emission arises
from a range of heights both above and below the chromospheric calcium
line, depending on the local temperature profile and electron densities.

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Title: Mimicking spectropolarimetric inversions using convolutional
neural networks
Authors: Milić, I.; Gafeira, R.
2020A&A...644A.129M Altcode: 2020arXiv200602005M
Context. Interpreting spectropolarimetric observations of the solar
atmosphere takes much longer than the acquiring the data. The most
important reason for this is that the model fitting, or "inversion",
used to infer physical quantities from the observations is extremely
slow, because the underlying models are numerically demanding. 
Aims: We aim to improve the speed of the inference by using a neural
network that relates input polarized spectra to the output physical
parameters. Methods: We first select a subset of the data
to be interpreted and infer physical quantities from corresponding
spectra using a standard minimization-based inversion code. Taking
these results as reliable and representative of the whole data set, we
train a convolutional neural network to connect the input polarized
spectra to the output physical parameters (nodes, in context of
spectropolarimetric inversion). We then apply the neural network to
the various other data, previously unseen to the network. As a check,
we apply the referent inversion code to the unseen data and compare
the fit quality and the maps of the inferred parameters between the
two inversions. Results: The physical parameters inferred by
the neural network show excellent agreement with the results from
the inversion, and are obtained in a factor of 105 less
time. Additionally, substituting the results of the neural network back
in the forward model, shows excellent agreement between inferred and
original spectra. Conclusions: The method we present here is
very simple for implementation and extremely fast. It only requires a
training data set, which can be obtained by inverting a representative
subset of the observed data. Applying these (and similar) machine
learning techniques will yield orders of magnitude acceleration in
the routine interpretation of spectropolarimetric data.

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Title: Chromospheric Resonances above Sunspots and Potential
Seismological Applications
Authors: Felipe, Tobias; Kuckein, Christoph; González Manrique,
Sergio Javier; Milic, Ivan; Sangeetha, C. R.
2020ApJ...900L..29F Altcode: 2020arXiv200810623F
Oscillations in sunspot umbrae exhibit remarkable differences
between the photosphere and chromosphere. We evaluate two competing
scenarios proposed for explaining those observations: a chromospheric
resonant cavity and waves traveling from the photosphere to upper
atmospheric layers. We have employed numerical simulations to
analyze the oscillations in both models. They have been compared with
observations in the low (Na I D2) and high (He I 10830 Å)
chromosphere. The nodes of the resonant cavity can be detected as
phase jumps or power dips, although the identification of the latter
is not sufficient to claim the existence of resonances. In contrast,
phase differences between velocity and temperature fluctuations reveal
standing waves and unequivocally prove the presence of an acoustic
resonator above umbrae. Our findings offer a new seismic method to probe
active region chromospheres through the detection of resonant nodes.

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Title: Chromospheric resonant cavities in umbrae: unequivocal
detection and seismic applications
Authors: Felipe, T.; Kuckein, C.; González Manrique, S. J.; Milic,
I.; Sangeetha, C. R.
2020sea..confE.196F Altcode:
Umbral chromospheric oscillations exhibit significant differences
compared to their photospheric counterparts. We evaluate two competing
scenarios proposed for explaining those observations: a chromospheric
resonant cavity and waves traveling from the photosphere to upper
atmospheric layers. The oscillatory signatures of both models have been
determined from numerical simulations, and they have been compared to
observations. We find that a high-frequency peak in the He I 10830 Å
power spectra cannot discriminate between both theories, contrary to the
claims of Jess et al. (2019). In contrast, phase differences between
velocity and temperature fluctuations reveal a standing pattern and
unequivocally prove the presence of an acoustic cavity above umbrae. Our
findings offer a new seismic method to probe sunspot chromospheres
through the identification of resonant nodes in phase spectra.

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Title: Spectral deconvolution with deep learning: removing the
effects of spectral PSF broadening
Authors: Molnar, Momchil; Reardon, Kevin P.; Osborne, Christopher;
Milić, Ivan
2020FrASS...7...29M Altcode: 2020arXiv200505529M
We explore novel methods of recovering the original spectral line
profiles from data obtained by instruments that sample those profiles
with an extended or multipeaked spectral transmission profile. The
techniques are tested on data obtained at high spatial resolution from
the Fast Imaging Solar Spectrograph (FISS) grating spectrograph at the
Big Bear Solar Observatory and from the Interferometric Bidimensional
Spectrometer (IBIS) instrument at the Dunn Solar Telescope. The method
robustly deconvolves wide spectral transmission profiles for fields of
view sampling a variety of solar structures (granulation, plage and
pores) with a photometrical precision of less than 1%. The results
and fidelity of the method are tested on data from IBIS obtained
using several different spectral resolution modes. The method, based
on convolutional neural networks (CNN), is extremely fast, performing
about 10^5 deconvolutions per second on a single CPU for a spectrum with
40 wavelength samples. This approach is applicable for deconvolving
large amounts of data from instruments with wide spectral profiles,
such as the Visible Tunable Filter (VTF) on the DKI Solar Telescope
(DKIST). We also investigate the application to future instruments
by recovering spectral line profiles obtained with a theoretical
multi-peaked spectral transmission profile. We further discuss the
limitations of this deconvolutional approach through the analysis of
the dimensionality of the original and multiplexed data.

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Title: Sun-as-a-star observations of the 2017 August 21 solar eclipse
Authors: Dineva, Ekaterina; Denker, Carsten; Verma, Meetu; Strassmeier,
Klaus G.; Ilyin, Ilya; Milic, Ivan
2020IAUS..354..473D Altcode:
The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI)
is a state-of-the-art, thermally stabilized, fiber-fed, high-resolution
spectrograph for the Large Binocular Telescope (LBT) at Mt. Graham,
Arizona. During daytime the instrument is fed with sunlight from the
10-millimeter aperture, fully automated, binocular Solar Disk-Integrated
(SDI) telescope. The observed Sun-as-a-star spectra contain a multitude
of photospheric and chromospheric spectral lines in the wavelength
ranges 4200-4800 Å and 5300-6300 Å. One of the advantages of PEPSI
is that solar spectra are recorded in the exactly same manner as
nighttime targets. Thus, solar and stellar spectra can be directly
compared. PEPSI/SDI recorded 116 Sun-as-a-star spectra during the
2017 August 21 solar eclipse. The observed maximum obscuration was
61.6%. The spectra were taken with a spectral resolution of ≈ 250000
and an exposure time of 0.3 s. The high-spectral resolution facilitates
detecting subtle changes in the spectra while the Moon passes the solar
disk. Sun-as-a-star spectra are affected by changing contributions due
to limb darkening and solar differential rotation, and to a lesser
extend by supergranular velocity pattern and the presence of active
regions on the solar surface. The goal of this study is to investigate
the temporal evolution of the chromospheric Na D doublet during the
eclipse and to compare observations with synthetic line profiles
computed with the state-of-the-art Bifrost code.

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Title: Using the infrared iron lines to probe solar subsurface
convection
Authors: Milić, I.; Smitha, H. N.; Lagg, A.
2019A&A...630A.133M Altcode: 2019arXiv190407306M
Context. Studying the properties of solar convection using
high-resolution spectropolarimetry began in the early 1990s with
the focus on observations in the visible wavelength regions. Its
extension to the infrared (IR) remains largely unexplored. 
Aims: The IR iron lines around 15 600 Å, most commonly known for
their high magnetic sensitivity, also have a non-zero response to
line-of-sight (LOS) velocity below log(τ) = 0.0. In this paper we
explore the possibility of using these lines to measure subsurface
convective velocities. Methods: By assuming a snapshot of a
three-dimensional magnetohydrodynamic simulation to represent the quiet
Sun, we investigate how well the iron IR lines can reproduce the LOS
velocity in the cube and to what depth. We use the recently developed
spectropolarimetric inversion code SNAPI and discuss the optimal node
placements for the retrieval of reliable results from these spectral
lines. Results: We find that the IR iron lines can measure the
convective velocities down to log(τ) = 0.5, below the photosphere,
not only at the original resolution of the cube, but also when degraded
with a reasonable spectral and spatial PSF and stray light. Instead, the
commonly used Fe I 6300 Å line pair performs significantly worse. Conclusions: Our investigation reveals that the IR iron lines can
probe the subsurface convection in the solar photosphere. This paper
is a first step towards exploiting this diagnostic potential.

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Title: The specific property of motion of resonant asteroids with
very slow Yarkovsky drift speeds
Authors: Milić; Žitnik, Ivana
2019MNRAS.486.2435M Altcode:
This work examines the specific characteristics of the motion of
asteroids with very slow Yarkovsky drift speeds (da/dt) across the
two-body mean-motion resonances (MMRs) with Jupiter, whose strengths
cover a wide range. Only asteroids that crossed a resonance completely
were observed. The investigation was carried out using numerical
integrations performed with the public-domain integrator ORBIT9. It was
found that the test asteroids with very small Yarkovsky drift speeds
moved extremely rapidly across MMRs (order of magnitude 10-5
au Myr-1 or less). This result may indicate that, below
a certain boundary value of da/dt, asteroids typically move quickly
across MMRs. From the obtained results, it is concluded that the
boundary value of the Yarkovsky drift speed is 7 × 10-5
au Myr-1.

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Title: Mimicking spectropolarimetric inversion using convolutional
neural networks
Authors: Milic, Ivan; Gafeira, Ricardo
2019AAS...23422605M Altcode:
State of the art approach for the interpretation of
spectropolarimetric observations of the solar atmosphere are the so
called spectropolarimetric inversions. These methods fit a model
atmosphere to the observed polarized spectrum and provide us with
the maximum-likelyhood solution for the parameters of the underlying
atmosphere. Inversions are extremely numerically demanding, because
they fully take into account all the physical processes involved in
the spectral line formation. This is especially pronounced in the case
of spectral lines formed in the solar chromosphere. With the advent of
next generation telescopes, such as DKIST, standard, minimization-based,
inversions will simply be too slow. In this contribution we propose
a way to accelerate the inversions by means of convolutional neural
networks. We invert a small sub-set of the data using standard
inversion approach and then train a convolutional neural network to
generalize the results to the full data set. We analyze this method
on different synthetic and observed data sets and compare the results
with the results obtained by applying standard inversion methods. We
find that, given an extensive enough data set, convolutional neural
networks provide results that are very close to the ones obtained by
standard inversion methods, in a fraction of time.

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Title: Center-to-Limb Continuum Polarization in Solar and Stellar
Atmospheres
Authors: Kostogryz, N. M.; Berdyugina, S. V.; Yakobchuk, T. M.;
Milić, I.
2019ASPC..526..139K Altcode:
The center-to-limb variation of the intensity (CLVI) and of the linear
polarization (CLVP) of stellar radiation arise when the scattering
and absorption processes are important in the stellar atmosphere. We
model the CLVI and CLVP of continuum radiation, taking into account
different contributions of scattering and absorption opacity for
a variety of spectral type stars with plane-parallel and spherical
PHOENIX atmosphere models. We show how the polarization depends on
the effective temperature and surface gravity of a star and how the
considered geometry of the stellar atmosphere affects the polarization
signal. For the Sun, we compare existing measurements with our
theoretical predictions for different solar models (FALA, FALC, FALP,
HSRA, and Phoenix). The CLVI and CLVP of stellar atmospheres are also
needed to interpret the light curves of transiting exoplanets. Here we
present the variation of the polarization in exoplanetary systems caused
by transits and grazing transits and discuss how the considered geometry
of stellar atmosphere models affect the transit curves of exoplanets.

---------------------------------------------------------
Title: Response Functions for NLTE Lines
Authors: Milić, I.; van Noort, M.
2019ASPC..526..179M Altcode:
Response functions quantify the sensitivity of the emergent polarized
spectrum to perturbations in the atmospheric quantities. They are
an important diagnostics tool and an essential ingredient of the
so-called inversion codes, widely used in solar spectropolarimetry. The
computation of response functions for spectral lines formed out of
local thermodynamic equilibrium is complicated because of strong
spatial and non-linear couplings of the atomic populations. We have
recently proposed a novel, analytic approach for the computation of
NLTE response functions, and in this short contribution we discuss
the possibilities of computing response functions for scattering
polarization. We explicitly show the procedure for a two level atom
line (normal Zeeman triplet), and discuss the "formation heights"
of intensity and scattering polarization for a prototype line.

---------------------------------------------------------
Title: Department of astronomy at Petnica science center: 2013-2017
Authors: Boskovic, M.; Obuljen, A.; Vukadinovic, D.; Milosevic, S.;
Milic, I.; Bozic, N.
2018POBeo..98..101B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spectropolarimetric NLTE inversion code SNAPI
Authors: Milić, I.; van Noort, M.
2018A&A...617A..24M Altcode: 2018arXiv180608134M
Context. Inversion codes are computer programs that fit a model
atmosphere to the observed Stokes spectra, thus retrieving the relevant
atmospheric parameters. The rising interest in the solar chromosphere,
where spectral lines are formed by scattering, requires developing,
testing, and comparing new non-local thermal equilibrium (NLTE)
inversion codes. Aims: We present a new NLTE inversion code that
is based on the analytical computation of the response functions. We
named the code SNAPI, which is short for spectropolarimetic NLTE
analytically powered inversion. Methods: SNAPI inverts full
Stokes spectrum in order to obtain a depth-dependent stratification of
the temperature, velocity, and the magnetic field vector. It is based
on the so-called node approach, where atmospheric parameters are free
to vary in several fixed points in the atmosphere, and are assumed to
behave as splines in between. We describe the inversion approach in
general and the specific choices we have made in the implementation. Results: We test the performance on one academic problem and on two
interesting NLTE examples, the Ca II 8542 and Na I D spectral lines. The
code is found to have excellent convergence properties and outperforms
a finite-difference based code in this specific implementation by at
least a factor of three. We invert synthetic observations of Na lines
from a small part of a simulated solar atmosphere and conclude that
the Na lines reliably retrieve the magnetic field and velocity in the
range -3 &lt; logτ &lt; -0.5.

---------------------------------------------------------
Title: Sweep-by-sweep implicit Lambda iteration for non-LTE radiative
transfer in 2D Cartesian coordinates
Authors: Milic, I.; Atanackovic, O.
2017POBeo..96..147M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Polarimetry of transiting planets: Differences between
plane-parallel and spherical host star atmosphere models
Authors: Kostogryz, N. M.; Yakobchuk, T. M.; Berdyugina, S. V.;
Milic, I.
2017A&A...601A...6K Altcode:
Context. To properly interpret photometric and polarimetric observations
of exoplanetary transits, accurate calculations of center-to-limb
variations of intensity and linear polarization of the host star are
needed. These variations, in turn, depend on the choice of geometry of
stellar atmosphere. Aims: We want to understand the dependence
of the flux and the polarization curves during a transit on the choice
of the applied approximation for the stellar atmosphere: spherical
and plane-parallel. We examine whether simpler plane-parallel models
of stellar atmospheres are good enough to interpret the flux and the
polarization light curves during planetary transits, or whether more
complicated spherical models should be used. Methods: Linear
polarization during a transit appears because a planet eclipses
a stellar disk and thus breaks left-right symmetry. We calculate
the flux and the polarization variations during a transit with given
center-to-limb variations of intensity and polarization. Results:
We calculate the flux and the polarization variations during transit for
a sample of 405 extrasolar systems. Most of them show higher transit
polarization for the spherical stellar atmosphere. Our calculations
reveal a group of exoplanetary systems that demonstrates lower maximum
polarization during the transits with spherical model atmospheres of
host stars with effective temperatures of Teff = 4400-5400
K and surface gravity of log g = 4.45-4.65 than that obtained with
plane-parallel atmospheres. Moreover, we have found two trends of the
transit polarization. The first trend is a decrease in the polarization
calculated with spherical model atmosphere of host stars with effective
temperatures Teff = 3500-5100 K, and the second shows an
increase in the polarization for host stars with Teff =
5100-7000 K. These trends can be explained by the relative variation
of temperature and pressure dependences in the plane-parallel and
spherical model atmospheres. Conclusions: For most cases of
known transiting systems the plane-parallel approximation of stellar
model atmospheres may be safely used for calculation of the flux and
the polarization curves because the difference between two models
is tiny. However, there are some examples where the spherical model
atmospheres are necessary to get proper results, such as the systems
with grazing transits, with Earth-size planets, or for the hot host
stars with effective temperatures higher than 6000 K.

---------------------------------------------------------
Title: Line response functions in nonlocal thermodynamic
equilibrium. Isotropic case
Authors: Milić, I.; van Noort, M.
2017A&A...601A.100M Altcode:
Context. Response functions provide us with a quantitative measure
of sensitivity of the emergent spectrum to perturbations in the
solar atmosphere and are thus the method of choice for interpreting
spectropolarimetric observations. For the lines formed in the solar
chromosphere, it is necessary to compute these responses taking into
account nonlocal thermodynamic equilibrium (NLTE) effects. 
Aims: We show how to analytically compute the response of the level
populations in NLTE to a change of a given physical quantity at a
given depth in the atmosphere. These responses are then used to compute
opacity and emissivity responses, which are then propagated to obtain
the response of the emergent intensity. Methods: Our method is
based on the derivative of the rate equations, where we explicitly
incorporate spatial coupling in the radiative rate terms. After
considering and collecting all interdependencies, the problem reduces
to a linear system of equations with a dimension equal to the product
of the number of spatial points and the number of energy levels. Results: We compare analytically computed response functions with
those obtained using a finite difference approach and find very good
agreement. In addition, a more accurate way of propagating opacity
and emissivity perturbations through the numerical solution of the
radiative transfer equation was developed. Conclusions: This
method allows for the fast evaluation of the response of the emergent
spectrum to perturbations of a given quantity at a given depth, and
thus is a significant step towards more efficient NLTE inversions.

---------------------------------------------------------
Title: Inference of magnetic fields in inhomogeneous prominences
Authors: Milić, I.; Faurobert, M.; Atanacković, O.
2017A&A...597A..31M Altcode: 2016A&A...597A..31M; 2016arXiv160904954M
Context. Most of the quantitative information about the magnetic
field vector in solar prominences comes from the analysis of the
Hanle effect acting on lines formed by scattering. As these lines can
be of non-negligible optical thickness, it is of interest to study
the line formation process further. Aims: We investigate the
multidimensional effects on the interpretation of spectropolarimetric
observations, particularly on the inference of the magnetic field
vector. We do this by analyzing the differences between multidimensional
models, which involve fully self-consistent radiative transfer
computations in the presence of spatial inhomogeneities and velocity
fields, and those which rely on simple one-dimensional geometry. Methods: We study the formation of a prototype line in ad hoc
inhomogeneous, isothermal 2D prominence models. We solve the NLTE
polarized line formation problem in the presence of a large-scale
oriented magnetic field. The resulting polarized line profiles are
then interpreted (I.e. inverted) assuming a simple 1D slab model. Results: We find that differences between input and the inferred
magnetic field vector are non-negligible. Namely, we almost universally
find that the inferred field is weaker and more horizontal than the
input field. Conclusions: Spatial inhomogeneities and radiative
transfer have a strong effect on scattering line polarization in the
optically thick lines. In real-life situations, ignoring these effects
could lead to a serious misinterpretation of spectropolarimetric
observations of chromospheric objects such as prominences.

---------------------------------------------------------
Title: Response functions for NLTE lines
Authors: Milic, Ivan
2017psio.confE..22M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Center-to-limb variation of intensity and polarization in
continuum spectra of FGK stars for spherical atmospheres
Authors: Kostogryz, N. M.; Milic, I.; Berdyugina, S. V.; Hauschildt,
P. H.
2016A&A...586A..87K Altcode: 2015arXiv151107213K
 Aims: One of the necessary parameters needed for the
interpretation of the light curves of transiting exoplanets or
eclipsing binary stars (as well as interferometric measurements of a
star or microlensing events) is how the intensity and polarization
of light changes from the center to the limb of a star. Scattering
and absorption processes in the stellar atmosphere affect both the
center-to-limb variation of intensity (CLVI) and polarization (CLVP). In
this paper, we present a study of the CLVI and CLVP in continuum
spectra, taking into consideration the different contributions of
scattering and absorption opacity for a variety of spectral type stars
with spherical atmospheres. Methods: We solve the radiative
transfer equation for polarized light in the presence of a continuum
scattering, taking into consideration the spherical model of a stellar
atmosphere. To cross-check our results, we developed two independent
codes that are based on Feautrier and short characteristics methods,
respectively, Results: We calculate the center-to-limb variation
of intensity (CLVI) and polarization (CLVP) in continuum for the
Phoenix grid of spherical stellar model atmospheres for a range of
effective temperatures (4000-7000 K), gravities (log g = 1.0-5.5), and
wavelengths (4000-7000 Å), which are tabulated and available at the
CDS. In addition, we present several tests of our codes and compare our
calculations for the solar atmosphere with published photometric and
polarimetric measurements. We also show that our two codes provide
similar results in all considered cases. Conclusions: For
sub-giant and dwarf stars (log g = 3.0-4.5), the lower gravity and
lower effective temperature of a star lead to higher limb polarization
of the star. For giant and supergiant stars (log g = 1.0-2.5), the
highest effective temperature yields the largest polarization. By
decreasing the effective temperature of a star down to 4500-5500 K
(depending on log g), the limb polarization decreases and reaches a
local minimum. It increases again with a corresponding decrease in
temperature down to 4000 K. For the most compact dwarf stars (log g =
5.0-5.5), the limb polarization degree shows a maximum for models with
effective temperatures in the range 4200-4600 K (depending on log g) and
decreases toward higher and lower temperatures. The intensity and
polarization profiles are only available at the CDS via anonymous ftp to
<A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A87">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A87</A>

---------------------------------------------------------
Title: VizieR Online Data Catalog: Center-to-limb polarization of
FGK stars (Kostogryz+, 2016)
Authors: Kostogryz, N. M.; Milic, I.; Berdyugina, S. V.; Hauschildt,
P. H.
2016yCat..35860087K Altcode:
Calculated center-to-limb variations of intensity (limb darkening)
for different stellar parameters with spherical atmosphere model
approximation. All intensities are normalized to the intensity in
the center of stellar disks, therefore, in the centre of the disks
(mu=1.0), I(mu)/I(1.0)=1.0. The parameters of spherical Phoenix
stellar model atmospheres we considered here are the following:
effective temperature is in the range of 4000K-7000K with the step of
100K and for logg=1.0-5.5 with the step of 0.5. All calculations are
made for such wavelengths: 4000Å, 4500Å, 5000Å, 6000Å, 7000Å. We
also present the position of the stellar limb and calculated stellar
radius. Calculated center-to-limb variations of polarization
(CLVP) for different stellar parameters with spherical atmosphere model
approximation. All intensities are normalized to the intensity in the
center of stellar disks, therefore, in the centre of the disks (mu=1.0),
I(mu)/I(1.0)=1.0. The parameters of spherical Phoenix model atmosphere
we considered here are the following: effective temperature is in
the range of 4000K-7000 K with the step of 100K and for logg=1.0-5.5
with the step of 0.5. All calculations are made for such wavelengths:
4000Å, 4500Å, 5000Å, 6000Å, 7000Å. We also present the position
of the stellar limb. (2 data files).

---------------------------------------------------------
Title: Inhomogeneity and velocity fields effects on scattering
polarization in solar prominences
Authors: Milić, I.; Faurobert, M.
2015IAUS..305..238M Altcode:
One of the methods for diagnosing vector magnetic fields in solar
prominences is the so called "inversion" of observed polarized spectral
lines. This inversion usually assumes a fairly simple generative model
and in this contribution we aim to study the possible systematic errors
that are introduced by this assumption. On two-dimensional toy model
of a prominence, we first demonstrate importance of multidimensional
radiative transfer and horizontal inhomogeneities. These are able to
induce a significant level of polarization in Stokes U, without the
need for the magnetic field. We then compute emergent Stokes spectrum
from a prominence which is pervaded by the vector magnetic field and
use a simple, one-dimensional model to interpret these synthetic
observations. We find that inferred values for the magnetic field
vector generally differ from the original ones. Most importantly,
the magnetic field might seem more inclined than it really is.

---------------------------------------------------------
Title: Scattering line polarization in rotating, optically thick disks
Authors: Milić, I.; Faurobert, M.
2014A&A...571A..79M Altcode: 2014arXiv1409.2654M
Context. To interpret observations of astrophysical disks, it is
essential to understand the formation process of the emitted light. If
the disk is optically thick, scattering dominated and permeated by a
Keplerian velocity field, non-local thermodynamic equilibrium (NLTE)
radiative transfer modeling must be done to compute the emergent
spectrum from a given disk model. Aims: We investigate NLTE
polarized line formation in different simple disk models and aim to
demonstrate the importance of both radiative transfer effects and
scattering, as well as the effects of velocity fields. Methods:
We self-consistently solve the coupled equations of radiative transfer
and statistical equilibrium for a two-level atom model by means of
Jacobi iteration. We use the short characteristics method of formal
solution in two-dimensional axisymmetric media and compute scattering
polarization, that is Q/I and U/I line profiles, using the reduced
intensity formalism. We account for the presence of Keplerian velocity
fields by casting the radiative transfer equation in the observer's
frame. Results: Relatively simple (homogeneous and isothermal)
disk models show complex intensity profiles that owe their shape
to the interplay of multidimensional NLTE radiative transfer and
the presence of rotation. The degree of scattering polarization is
significantly influenced not only by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and
the presence of rotation. Stokes U/I shows double-lobed profiles with
amplitude that increases with the disk rotation. Conclusions: Our
results suggest that the line profiles, especially the polarized ones,
emerging from gaseous disks differ significantly from the profiles
predicted by simple approximations. Even in the case of the simple
two-level atom model, we obtain line profiles that are diverse in shape,
but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which
might prove to be a useful diagnostic tool. We also demonstrate that,
for moderate rotational velocities, an approximate treatment can be
used, where NLTE radiative transfer is done in the velocity field-free
approximation, and Doppler shift is applied in the process of spatial
integration over the whole emitting surface.

---------------------------------------------------------
Title: Inferring Depth-Dependent Quiet Sun Magnetic Fields
Authors: Milić, I.; Faurober, M.,
2014ASPC..489..157M Altcode:
Hanle effect has an invaluable diagnostic value, allowing us to diagnose
weak solar magnetic fields from the measure of the depolarization of
lines formed in scattering processes. Here we consider two molecular
lines which have been studied in the last decade: MgH and C2
lines. Different studies invoke different strengths of the magnetic
fields, ranging from ≍ 10 to ≍ 100 Gauss. We try to simultaneously
explain all this results with the assumption of depth-dependent
magnetic field with large depth gradient. Our inversion method,
which relies on the inversion of the center-to-limb variation (CLV)
of line-center polarization, can simultaneously explain observed CLV
in MgH and C2 lines, but yields very strong gradient of the
magnetic field strength. We conclude that a possible way to make this
kind of diagnostics more reliable is to combine it with multidimensional
modeling of Hanle effect in molecular and atomic lines.

---------------------------------------------------------
Title: Accelerating NLTE radiative transfer by means of the
Forth-and-Back Implicit Lambda Iteration: A two-level atom line
formation in 2D Cartesian coordinates
Authors: Milić, Ivan; Atanacković, Olga
2014AdSpR..54.1297M Altcode: 2014arXiv1401.4562M
State-of-the-art methods in multidimensional NLTE radiative transfer
are based on the use of local approximate lambda operator within either
Jacobi or Gauss-Seidel iterative schemes. Here we propose another
approach to the solution of 2D NLTE RT problems, Forth-and-Back Implicit
Lambda Iteration (FBILI), developed earlier for 1D geometry. In order
to present the method and examine its convergence properties we use the
well-known instance of the two-level atom line formation with complete
frequency redistribution. In the formal solution of the RT equation
we employ short characteristics with two-point algorithm. Using an
implicit representation of the source function in the computation
of the specific intensities, we compute and store the coefficients
of the linear relations J=a+bS between the mean intensity J and
the corresponding source function S. The use of iteration factors
in the ‘local’ coefficients of these implicit relations in two
‘inward’ sweeps of 2D grid, along with the update of the source
function in other two ‘outward’ sweeps leads to four times faster
solution than the Jacobi’s one. Moreover, the update made in all
four consecutive sweeps of the grid leads to an acceleration by a
factor of 6-7 compared to the Jacobi iterative scheme.

---------------------------------------------------------
Title: On some dynamical properties of Phocaea asteroids
Authors: Milić, I. S.; Novaković, B.
2014acm..conf..358M Altcode:
The Phocaea group is located in the inner asteroid belt, and consists
of asteroids having orbital inclination higher than about 20 degrees,
and eccentricity ranging between 0.15 and 0.3. This region is known to
have dynamical boundaries which completely surround it and produce a
confinement of objects inside it [1,2]. An inner boundary in semi-major
axis, often related to the 7/2 mean-motion resonance (MMR) with Jupiter,
is located at about 2.25 au. However, a non-negligible number of
asteroids is still present beyond this limit. The outer boundary, at
around 2.5 au, is set by the powerful 3/1 MMR with Jupiter. Moreover,
the region is delimited by important secular resonances (SRs):
the ν_{6}=g-g_{6} at low inclination, and the ν_{5}=g-g_{5} and
ν_{16}=s-s_{6} at high inclination. Also, the asteroids from the
region interact with many mean-motion and secular resonances. The most
relevant SRs are g-g_{6} - s + s_{6}, g - g_{5} + s - s_{6} and g -
g_{6} - 2s + 2s_{6}, as can be recognized by the alignments of objects
associated with these resonances (see Figure). The aim of this work
is twofold. First, to investigate the possible relevance of secular
resonances with the inner planets for the Phocaea asteroids. Second,
to check weather or not the inner boundary of the region in terms of
semi-major axis, i.e. 7/2 resonance with Jupiter, could be crossed under
the influence of gravitational and/or non-gravitational forces. Our
results confirm the non-negligible importance of secular resonances
involving inner planets for the dynamics over long time scales of
the asteroids in the Phocaea region. The most obvious interaction is
found with the s - s_{4} + g_{3} - g_{7} resonance. The results for
the 7/2 resonance show that a significant fraction of the bodies
larger than about 600 m, and most of the bodies smaller than the
above limit, can transit across the resonance without being removed
from the Phocaea region. This means that, despite being effective in
pumping up asteroid eccentricities in this region, the 7/2 resonance
is not an absolute dynamical boundary for sufficiently small objects,
below some hundred meters in diameter.

---------------------------------------------------------
Title: Ubrzanje metoda za rešavanje problema prenosa polarizovanog
zračenja u više dimenzija i njihova primena

---------------------------------------------------------
Title: Ubrzanje
metoda za rešavanje problema prenosa polarizovanog zračenja u
više dimenzija i njihova primena

---------------------------------------------------------
Title: Acceleration of methods for
multidimensional polarized radiative transfer and their application;
Authors: Milic, Ivan
2014PhDT.......574M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Multidimensional and inhomogeneity effects on scattering
polarization in solar prominences
Authors: Milić, Ivan; Faurobert, Marianne
2014IAUS..300..453M Altcode:
Measurements of magnetic fields in solar prominences via Hanle effect
usually assume either single scattering approximation or simple,
one-dimensional, slab model in order to perform an inversion and find
the unknown magnitude and the orientation of the magnetic field from
spectropolarimetric observations. Here we perform self-consistent
NLTE modeling of scattering polarization in inhomogeneous 2D slab,
illuminated from its sides by the solar continuum radiation. We show
that even in the absence of a magnetic field, in the non-optically thin
regime, significant non-zero Stokes U is to be expected. Neglecting
these effects, in principle, could cause systematic errors in
spectropolarimetric inversions, in the case when the prominence is
optically thick.

---------------------------------------------------------
Title: Boundary conditions for polarized radiative transfer with
incident radiation
Authors: Faurobert, M.; Milić, I.; Atanacković, O.
2013A&A...559A..68F Altcode: 2013arXiv1309.4160F
Context. Polarized radiative transfer in the presence of scattering
in spectral lines and/or in continua may be cast in a reduced form for
six reduced components of the radiation field. In this formalism, the
six components of the reduced source function are angle-independent
quantities. It thus drastically reduces the storage requirement
of numerical codes and it is very well suited to solving polarized
non-local thermodynamic equilibrium radiative transfer problems in 3D
media. Aims: This approach encounters a fundamental problem
when the medium is illuminated by a polarized incident radiation,
because there is a priori no way of relating the known (and measurable)
Stokes parameters of the incident radiation to boundary conditions
for the reduced equations. The origin of this problem is that there
is no unique way of deriving the radiation-reduced components from its
Stokes parameters (only the inverse operation is clearly defined). The
method proposed here aims at enabling to work with arbitrary
incident radiation field (polarized or unpolarized). Methods:
In previous studies, an ad-hoc treatment of the boundary conditions,
applied to cases where the incident radiation is unpolarized, has been
used. In this paper, we show that it is possible to account for the
incident radiation in a rigorous way without any assumption on its
properties by expressing the radiation field as the sum of a directly
transmitted radiation and of a diffuse radiation. This approach was
first used by Chandrasekhar to solve the problem of diffuse reflection
by planetary atmospheres illuminated by their host star. Results:
The diffuse radiation field obeys a transfer equation with no incident
radiation that may be solved in the reduced form. The first scattering
of the incident radiation introduces primary creation terms in the six
components of the reduced source function. Once the reduced polarized
transfer problem is solved for the diffuse radiation field, its Stokes
parameters can be computed. The full radiation field is then obtained
by adding the directly transmitted radiation field computed in the
Stokes formalism. Conclusions: In the case of an unpolarized
incident radiation, the diffuse field approach allows us to validate
the previously introduced ad-hoc expressions. The diffuse field
approach however leads to more accurate computation of the source
terms in the case where the incident radiation is anisotropic. It
is the only possible approach when the incident radiation field is
polarized. We perform numerical computations of test cases, showing
that the emergent line-polarization may be significantly affected by
the polarization of the incident radiation.

---------------------------------------------------------
Title: Transfer of polarized line radiation in 2D cylindrical geometry
Authors: Milić, I.
2013A&A...555A.130M Altcode:
 Aims: This paper deals with multidimensional NLTE polarized
radiative transfer in the case of two level atom in the absence
of lower level polarization. We aim to develop an efficient and
robust method for 2D cylindrical geometry and to apply it to various
axi-symmetrical astrophysical objects such as rings, disks, rotating
stars, and solar prominences. Methods: We review the methods
of short characteristics and Jacobi iteration applied to axisymmetric
geometry. Then we demonstrate how to use a reduced basis for polarized
intensity and polarized source function to self-consistently solve the
coupled equations of radiative transfer and statistical equilibrium
for linearly polarized radiation. We discuss some peculiarities that
do not appear in Cartesian geometry, such as angular interpolation in
performing the formal solution. We also show how to account for two
different types of illuminating radiation. Results: The proposed
method is tested on homogeneous, self-emitting cylinders to compare
the results with those in 1D geometries. We demonstrate a possible
astrophysical application on a very simple model of circumstellar
ring illuminated by a host star where we show that such a disk can
introduce a significant amount of scattering polarization in the
system. Conclusions: This method is found to converge properly
and, apparently, to allow for substantial time saving compared to 3D
Cartesian geometry. We also discuss the advantages and disadvantages
of this approach in multidimensional radiative transfer modeling.

---------------------------------------------------------
Title: CCD Observations of ERS with the 60 cm Telescope at ASV
Authors: Damljanovic, G.; Milic, I. S.
2013POBeo..92..161D Altcode:
We present the observations of extragalactic radio sources (ERS) which
are possible in the optical domain and can be used to establish the
link between the ICRF2 and the future Gaia Celestial Reference Frame
(GCRF). Our telescope of small aperture size (&lt; 1 m) is located in
the south of Serbia, near the town of Prokuplje, at the Astronomical
Station Vidojevica (ASV) which belongs to the Astronomical Observatory
of Belgrade (AOB). It is a Cassegrain-type optical system (D=60
cm, F=600 cm) of equatorial mount. About 40 ERS, from ICRF2 list,
were observed at ASV during 2011 and 2012. These observations are
of importance to compare the ERS optical and radio positions (VLBI
ones), and to investigate the relation between optical and radio
reference frames. Also, they are useful to check the possibilities of
the instrument. We observed ERS with the CCD Apogeee Alta U42. The
observations, reduction and preliminary results of some ERS are
presented here.

---------------------------------------------------------
Title: MONECOM: Physical Characteristics Of Main Belt Comets
Authors: Bogdanovic, N.; Smolic, I.; Bogosavljevic, M.; Milic, I.
2013POBeo..92..153B Altcode:
The aim of the MONECOM project is to carry out photometric observations
of several Main-Belt Comets (MBCs). Observations and data reduction were
performed by high-school students from three countries (Croatia, Greece
and Serbia), supervised by their teachers and local astronomers. Here
we present some results obtained by the Serbian group.

---------------------------------------------------------
Title: Observational Astronomy at Petnica Science Center
Authors: Milic, I.; Obuljen, A.; Bozic, N.; Smolic, I.; Boskovic, M.
2013POBeo..92..185M Altcode:
During years 2012 and 2013, Petnica Science Center has been undergoing
a thorough renovation and expansion. One of the new features will
be new observational equipment intended for high-level educational
work at the Department of astronomy. In this short paper we sum up
main observational activities at Petnica Science Center, and discuss
desired observational equipment.

---------------------------------------------------------
Title: Astrometric Positions of ICRF2 Radio Sources with Different
Reference Catalogues
Authors: Damljanovic, G.; Milic, I. S.; Maigurova, N.; Martynov, M.;
Pejovic, N.
2012POBeo..91..191D Altcode:
We present the results of an investigation of astrometric positions of a
few extragalactic radio sources (ERS) from the ICRF2 list. The reference
systems are based on the resolutions of the international scientific
unions. The celestial system is based on IAU (International Astronomical
Union) Resolution A4 (1991). It was officially initiated and named
International Celestial Reference System (ICRS) by IAU Resolution B2
(1997). Its definition was further refined by IAU Resolution B1 (2000)
and Resolution B3 (2009). The fundamental celestial reference frame
(International Celestial Reference Frame -- ICRF) was adopted by the
IAU (1997), with its original list of radio objects and two extensions
(ICRF-ext1 and ICRF-ext2); hereafter referred to as ICRF1. Alltogether,
there were 717 sources: 212 defining ones, 109 new ones, 294 candidate
ones, and 102 additional sources. At the IAU XXVII GA (2009), the second
realization of the ICRF (the ICRF2) was adopted with the list of precise
positions for 3414 compact radio astronomical sources. It is more then
five times the number as in the ICRF1. At that moment there were nearly
30 years of VLBI (Very Long Baseline Interferometry) observations of
some radio sources. The ICRF2 has a noise threshold of about 0.04 mas
(nearly 6 times better than ICRF1) and an axis stability of about 0.01
mas (nearly twice as stable as ICRF1). Also, a search for a relation
between optical and radio reference frames is important. To do that,
it is necessary to make the observations of some ICRF2 ERS which
are visible in the optical domain, and to compare their optical and
radio positions (VLBI ones). The optical positions (α and δ) could
be calculated using reference stars from some of nowadays big star
catalogues. The XPM, 2MASS (with XC1) and DR7 SDSS ones were used here,
and the relative method was applied. We started to do that comparison
using our CCD observations of a few ERS made with the 2 m RCC telescope
(with the focal length of 16 m) of Rozhen National Astronomical
Observatory (Bulgarian Academy of Sciences). About 30 fields around
ERS were observed with CCD camera VersArray 1300B (1340x1300 pixels,
the pixel size is 20x20 micrometers, one pixel is 0.258 arcsec) in the
end of March 2011. The main steps of our reduction and preliminary
results are presented here. Based on observations with the 2 m
RCC telescope of the Rozhen National Astronomical Observatory operated
by the Institute of Astronomy, Bulgarian Academy of Sciences.

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Title: Resonance Line Polarization in Moving Optically Thick
Structures
Authors: Milic, I.; Faurobert, M.
2012POBeo..91...45M Altcode:
We compute the scattering polarization of lines formed in moving
slabs of moderate optical thickness (τ=1 and τ=10) illuminated
by a linearly polarized radiation field showing a broad absorption
feature. Slabs are one-dimensional and horizontal, placed at a finite
height H above a semi-infinite atmosphere. This model is an academic
case which represents the formation of emergent radiation in solar
filaments. The slabs have a macroscopic velocity in the radial direction
with respect to the atmosphere, and are observed at different angles
(i.e. at different locations over the solar disk). We investigate
the sensitivity of the outgoing polarization to the slab velocity and
observing angle. We show that outgoing polarization profiles are at
least as sensitive to macroscopic velocity as are intensity profiles.

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Title: CCD measurements in optical domain and astrometric positions
of ICRF2 radio sources
Authors: Damljanovic, G.; Milic, I. S.
2012jsrs.conf...92D Altcode:
At the IAU XXIIIth GA in 1997, the International Celestial Reference
Frame (ICRF) was adopted; hereafter referred to as ICRF1. After the
original list of radio objects there were two extensions, ICRF-ext1
and ICRF-ext2. All together, there were 717 sources: 212 defining ones,
109 new ones, 294 candidate ones, and 102 additional ones. At the IAU
XXVIIth GA in 2009, the second realization of the ICRF (the ICRF2)
was adopted with the list of precise positions for 3414 compact radio
astronomical sources. At that moment there were nearly 30 years of
VLBI observations. The ICRF2 has a noise floor of about 0.04 mas (near
six times better than ICRF1) and an axis stability of about 0.01 mas
(nearly twice as stable as ICRF1). Also, it is of importance to make the
observations of some ICRF2 extragalactic radio sources (ERS) which are
visible in the optical domain, and to compare their optical (calculated
via the reference stars) and radio positions (VLBI ones). We started to
do it by using the CCD camera VersArray 1300B and the RCC telescope1
(D/F = 2m/16m) of Rozhen National Astronomical Observatory (Bulgarian
Academy of Sciences). About 30 frames were observed at the end of March
2011. The main steps of our calculations and some preliminary results
(comparison between the measured optical positions and the radio ones)
for a few ERS from ICRF2 list were presented here.

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Title: Hanle diagnostics of weak solar magnetic fields: . Inversion
of scattering polarization in C2 and MgH molecular lines
Authors: Milić, I.; Faurobert, M.
2012A&A...547A..38M Altcode:
Context. The quiet Sun magnetism has been intensively investigated
in recent years by various observational techniques. But several
issues, such as the question of the isotropy and of the energy density
spectrum of the mixed polarity turbulent magnetic fields, are still
under debate. Aims: Here we present an inversion method that
allows us to constrain the depth-dependence of the magnetic field
strength. We use the center-to-limb variations of linear scattering
polarization measured in molecular lines of C2 and MgH
molecules with different sensitivities to the Hanle effect. We consider
six C2-triplets and one MgH line in the spectral range
between 515.7 nm and 516.1 nm observed with the THEMIS Telescope. Methods: One of the delicate problems with Hanle diagnostics is
to disentangle the effects of elastic depolarizing collisions from
the depolarization due to the Hanle effect of the magnetic field. By
making use of the different sensitivities of the molecular lines in
our spectral range to microturbulent magnetic fields and, by using
a non-LTE radiative transfer modeling of the line formation, we are
able to determine both the depolarizing collision cross-section and the
magnetic strength. We use a standard 1D quiet Sun atmospheric model and
we invert the full set of center-to-limb polarization rates measured
at line centers, with a depth-dependent magnetic field described by
three free parameters. The depolarizing collision cross-section is
also treated as a free parameter. A downhill simplex method is used to
find the best-fitting values for the collisional and magnetic strength
parameters. Results: For the elastic depolarizing collisions
cross-section for the C2 lines we obtain α(2)
= 1.6 ± 0.4 × 10-9 cm3 s-1, which
is within an order of magnitude of the value previously obtained for
MgH lines from a differential Hanle effect analysis. The observational
constraints provided by the MgH and C2 line polarization
give access to the altitude range between z = 200 km and z = 400 km
above the base of the photosphere. We find that the turbulent magnetic
field strength decreases from 95 Gauss at the altitude z = 200 km to 5
Gauss at z = 400 km. Conclusions: The turbulent magnetic field
strength that we derive from the Hanle effect shows a strong vertical
gradient in the upper photosphere. We point out that this behavior
may explain why very different turbulent magnetic field strengths
have been inferred from the interpretation of Hanle depolarization
when using different lines formed at different altitudes. We notice
that the presence of a strong depth gradient is not compatible with
the assumption of isotropy of the turbulent field.

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Title: Numerical modeling of the linear polarization in molecular
lines of the solar flash spectrum
Authors: Milić, I.; Faurobert, M.
2012EAS....55...65M Altcode:
Molecular lines formed in the upper photosphere of the Sun show
significant degree of linear scattering polarization, when one observes
close to the solar limb. Those lines prove to be important tools for
turbulent magnetic field diagnostic via the Hanle effect. In order to
correctly model the line formation in regions close to the Solar limb,
one has to take into account the sphericity of the atmosphere and to
model depolarizing collisions and NLTE line formation in details. We
present computations of scattering polarization in C2 lines in a 1D
spherically symmetric medium described by FALC and FALX models, and
compare those computations to observations of the Solar flash spectrum.

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Title: Focal Length Determination for the 60 cm Telescope at
Astronomical Station Vidojevica
Authors: Cvetkovic, Z.; Damljanovic, G.; Pavlovic, R.; Vince, O.;
Milic, I. S.; Stojanovic, M.
2012SerAJ.184...97C Altcode:
The focal length of a telescope is an important parameter in
determining the angular pixel size. This parameter is used for the
purpose of determining the relative coordinates (angular separation
and positional angle) of double and multiple stars, and the precise
coordinates of extragalactic radio sources (ERS) that are visible at
optical wavelengths. At the Astronomical Station Vidojevica we have
collected observations of these objects using two CCD cameras, Apogee
Alta U42 and SBIG ST-10ME, attached to the 60 cm telescope. Its nominal
focal length is 600 cm as given by the manufacturer. To determine the
telescope focal length more precisely for both attached detectors,
we used angular-separation measurements from CCD images taken at
Astronomical Station Vidojevica. The obtained focal lengths are: F_{42}
= (5989±7) mm using the CCD camera Apogee Alta U42 attached to the
telescope, and F_{10} = (5972±4) mm with the CCD camera SBIG ST-10ME
attached to the telescope.

---------------------------------------------------------
Title: Modeling scattering polarization in molecular solar lines in
spherical geometry
Authors: Milić, I.; Faurobert, M.
2012A&A...539A..10M Altcode:
Context. The atmosphere of the Sun is permeated by a vast amount
of magnetic flux that remains invisible in magnetograms based on the
Zeeman effect. A model-independent way of measuring weak hidden magnetic
fields makes use of the differential Hanle effect on the scattering
polarization of molecular lines with different sensitivities to magnetic
fields. Aims: The observed line scattering polarization steeply
increases at the solar limb. Here we are interested in interpreting
observations performed at the solar limb, where plane-parallel
semi-infinite geometry is not valid. The main reason is that the
sphericity of the atmosphere means that the line-of-sight optical path
intersects only a finite part of the solar atmosphere. In this paper
we revisit the modeling of scattering polarization in two molecular
lines of C2 and MgH in the spectral range from 515.60 nm to
516.20 nm, where observations performed both inside and above the solar
limb are available. Methods: The solar atmosphere is described
by a one-dimensional, spherically symmetric medium following either
the FALC or the FALX quiet Sun model. Both the line and background
continuum scattering polarizations are computed by means of the
"along-the-ray" approach. We assume a two-level atom formalism for
the line source function, and we compute the molecule number densities
and line opacities assuming LTE. We estimate the elastic and inelastic
collision rates by fitting the line intensity and linear polarization
in several couples of lines of the Second Solar Spectrum Atlas. Results: The limb variations of scattering polarization, both
in the lines and in the continuum, are strongly modified when the
sphericity of the solar atmosphere is accounted for. We show that the
line polarization goes through a maximum at 0.4” above the limb, for
both MgH and C2 lines. The contribution of the line rapidly
goes to zero at a larger limb distance, but continuum polarization
keeps increasing. The maximum polarization rates have an amplitude of
2% to 2.5% when the FALC model is used, which agrees with previous
observations, whereas the FALX model leads to much higher rates. We
then investigate the Hanle effect of microturbulent magnetic fields on
the C2 line linear polarization. We show that polarization
observed close to the limb would provide valuable diagnostics of weak
magnetic fields in the region of the temperature minimum.

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Title: Scattering polarization of molecular lines at the solar limb
Authors: Milić, I.; Faurobert, M.
2011sf2a.conf..401M Altcode:
Molecular lines formed in the upper photosphere of the Sun show linear
scattering polarization, when one observes close to the solar limb. This
provides us with a diagnostic tool for measuring weak magnetic fields
in the solar photosphere through the differential Hanle effect
in these lines. However, in order to interpret polarization ratio
measured in different lines of different optical thickness, one has
to model accurately enough the line formation processes. Observations
performed close to and above the solar limb give access to the still
poorly known region of the temperature minimum between the photosphere
and the chromosphere. The modeling of such observations requires to
account for the spherical geometry of the solar atmosphere. Here we
revisit the modeling of molecular solar line scattering polarization
in spherical geometry and we investigate its diagnostics potential.

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Title: VizieR Online Data Catalog: Corrected proper motion for HIP
stars (Damljanovic+, 2011)
Authors: Damljanovic, G.; Milic, I. S.
2011yCatp042018201D Altcode:
During the last century, there were many so-called independent latitude
(IL) stations with the observations which were included into data
of a few international organizations (like Bureau International de
l'Heure - BIH, International Polar Motion Service - IPMS) and the Earth
rotation programmes for determining the Earth Orientation Parameters
- EOP. Because of this, nowadays, there are numerous astrometric
ground-based observations (made over many decades) of some stars
included in the Hipparcos Catalogue (ESA 1997, Cat. I/239). We used
these latitude data for the inverse investigations - to improve the
proper motions in declination μδ of the mentioned Hipparcos
stars. We determined the corrections Δμδ and investigated
agreement of our μδ and those from the catalogues Hipparcos
and new Hipparcos (van Leeuwen 2007, Cat. I/311). To do this we used
the latitude variations of 7 stations (Belgrade, Blagoveschtschensk,
Irkutsk, Poltava, Pulkovo, Warsaw and Mizusawa), covering different
intervals in the period 1904.7-1992.0, obtained with 6 visual and 1
floating zenith telescopes (Mizusawa). On the other hand, with regard
that about two decades have elapsed since the Hipparcos ESA mission
observations (the epoch of Hipparcos catalogue is 1991.25), the error
of apparent places of Hipparcos stars has increased by nearly 20mas
because of proper motion errors. Also, the mission lasted less than
four years which was not enough for a sufficient accuracy of proper
motions of some stars (such as double or multiple ones). Our method
of calculation, and the calculated μδ for the common
IL/Hipparcos stars are presented here. We constructed an IL catalogue
of 1200 stars: there are 707 stars in the first part (with at least 20
years of IL observations) and 493 stars in the second one (less than
20 years). In the case of μδ of IL stars observed at some
stations (Blagoveschtschensk, Irkutsk, Mizusawa, Poltava and Pulkovo)
we find the formal errors less than the corresponding Hipparcos ones
and for some of them (stations Blagoveschtschensk and Irkutsk) even
less than the new Hipparcos ones. (1 data file).

---------------------------------------------------------
Title: Corrected mu_delta for Stars of Hipparcos Catalogue from
Independent Latitude Observations over Many Decades
Authors: Damljanovic, G.; Milic, I. S.
2011SerAJ.182...35D Altcode:
During the last century, there were many so-called independent latitude
(IL) stations with the observations which were included into data
of a few international organizations (like Bureau International de
l'Heure - BIH, International Polar Motion Service - IPMS) and the
Earth rotation programmes for determining the Earth Orientation
Parameters - EOP. Because of this, nowadays, there are numerous
astrometric ground-based observations (made over many decades) of some
stars included in the Hipparcos Catalogue (ESA 1997). We used these
latitude data for the inverse investigations - to improve the proper
motions in declination μ_{δ} of the mentioned Hipparcos stars. We
determined the corrections Δμ_{δ} and investigated agreement of our
μ_{δ} and those from the catalogues Hipparcos and new Hipparcos (van
Leeuwen 2007). To do this we used the latitude variations of 7 stations
(Belgrade, Blagoveschtschensk, Irkutsk, Poltava, Pulkovo, Warsaw and
Mizusawa), covering different intervals in the period 1904.7 - 1992.0,
obtained with 6 visual and 1 floating zenith telescopes (Mizusawa). On
the other hand, with regard that about two decades have elapsed
since the Hipparcos ESA mission observations (the epoch of Hipparcos
catalogue is 1991.25), the error of apparent places of Hipparcos stars
has increased by nearly 20 mas because of proper motion errors. Also,
the mission lasted less than four years which was not enough for a
sufficient accuracy of proper motions of some stars (such as double or
multiple ones). Our method of calculation, and the calculated μ_{δ}
for the common IL/Hipparcos stars are presented here. We constructed
an IL catalogue of 1200 stars: there are 707 stars in the first part
(with at least 20 years of IL observations) and 493 stars in the
second one (less than 20 years). In the case of μ_{&amp;delta};
of IL stars observed at some stations (Blagoveschtschensk, Irkutsk,
Mizusawa, Poltava and Pulkovo) we find the formal errors less than
the corresponding Hipparcos ones and for some of them (stations
Blagoveschtschensk and Irkutsk) even less than the new Hipparcos ones.

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Title: Correlations of Orbital Elements for Visual Double Stars
Authors: Milic, I. S.; Cvetkovic, Z.
2010SerAJ.181...69M Altcode:
In this paper, the authors examine the dependence of correlation
coefficients of orbital elements on the length of the orbital arc
covered by measurements, on measurements of different accuracies, and
on the number of measurements. The obtained correlation coefficients
for the orbital elements are found to decrease with the orbital arc
length covered by measurements, they are independent of the measurement
precision, and they do not depend on the number of measurements for long
arcs and they decrease with the number of measurements for short arcs.

---------------------------------------------------------
Title: Comparing Observed Prominence Spectra with Simple Models
Computed Using GHV Code
Authors: Milic, I.; Kotrc, P.
2010POBeo..90..163M Altcode:
Emission spectra in seven lines have been computed for 980 prominence
models using code originally developed by Gouttebrose, Heinzel and
Vial. Prominence is treated as isothermal, isobaric, 1-D slab, with
five input parameters determining outgoing radiation. Computed emission
spectra in Hα have been compared with observations from the Ondřejov
large solar spectrograph and some of the results are discussed.

---------------------------------------------------------
Title: On Estimation of the Optical Thickness of Solar Prominences
Authors: Milic, I.; Dejanic, S.; Kotrc, P.
2009POBeo..86..283M Altcode:
A method for rough estimation of the optical thickness of solar
prominences in Hα line is presented. The method is based on the
fitting of observed profiles with the synthetic ones computed by using
the model of an isobaric 1-D slab with constant source function under
the assumption of complete redistribution. The method was applied on
52 prominences observed with the Ondřejov HSFA2 spectrograph from
April 2007 to March 2008.