explanation blue bibcodes open ADS page with paths to full text
Author name code: vitas
ADS astronomy entries on 2022-09-14
author:"Vitas, Nikola"
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Title: Modeling the thermal conduction in the solar atmosphere with
the code MANCHA3D
Authors: Navarro, A.; Khomenko, E.; Modestov, M.; Vitas, N.
2022A&A...663A..96N Altcode: 2022arXiv220508846N
Context. Thermal conductivity is one of the important mechanisms of
heat transfer in the solar corona. In the limit of strongly magnetized
plasma, it is typically modeled by Spitzer's expression where the
heat flux is aligned with the magnetic field. <BR /> Aims: This paper
describes the implementation of the heat conduction into the code
MANCHA3D with an aim of extending single-fluid MHD simulations from
the upper convection zone into the solar corona. <BR /> Methods:
Two different schemes to model heat conduction are implemented:
(1) a standard scheme where a parabolic term is added to the energy
equation, and (2) a scheme where the hyperbolic heat flux equation is
solved. <BR /> Results: The first scheme limits the time step due to the
explicit integration of a parabolic term, which makes the simulations
computationally expensive. The second scheme solves the limitations
on the time step by artificially limiting the heat conduction speed
to computationally manageable values. The validation of both schemes
is carried out with standard tests in one, two, and three spatial
dimensions. Furthermore, we implement the model for heat flux derived by
Braginskii (1965, Reviews of Plasma Physics, 205) in its most general
form, when the expression for the heat flux depends on the ratio of
the collisional to cyclotron frequencies of the plasma, and, therefore
on the magnetic field strength. Additionally, our implementation
takes into account the heat conduction in parallel, perpendicular,
and transverse directions, and provides the contributions from ions
and electrons separately. The model recovers Spitzer's expression
for parallel thermal conductivity in the strongly magnetized limit
but also transitions smoothly between field-aligned conductivity
and isotropic conductivity for regions with a low or null magnetic
field. We describe the details of the implementation of Braginskii's
thermal conductivity using a combination of the first scheme for the
perpendicular and transverse directions and the second scheme for the
parallel component. We estimate thermal conductivities in a quiet-Sun
model. In this model, we find that the perpendicular and transverse
components for electrons and ions and the parallel component for
ions might have some significance below the transition region. Above
the transition region only the parallel component for ions might
be important. Finally, we present a two-dimensional test for heat
conduction using realistic values of the solar atmosphere where we
prove the robustness of the two schemes implemented and show that our
adaptation of the hyperbolic treatment offers a great advantage over
the computational cost of the simulations.
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Title: Newly formed downflow lanes in exploding granules in the
solar photosphere
Authors: Ellwarth, M.; Fischer, C. E.; Vitas, N.; Schmiz, S.;
Schmidt, W.
2021A&A...653A..96E Altcode: 2021arXiv210700582E
Context. Exploding granules have drawn renewed interest because of
their interaction with the magnetic field (either emerging or already
present). Especially the newly forming downflow lanes developing in
their centre seem to be eligible candidates for the intensification
of magnetic fields. We analyse spectroscopic data from two different
instruments in order to study the intricate velocity pattern within
the newly forming downflow lanes in detail. <BR /> Aims: We aim to
examine general properties of a number of exploding granules, such
as their lifetime and extend. To gain a better understanding of the
formation process of the developing intergranular lane in exploding
granules, we study the temporal evolution and height dependence of the
line-of-sight velocities at their formation location. Additionally, we
search for evidence that exploding granules act as acoustic sources. <BR
/> Methods: We investigated the evolution of several exploding granules
using data taken with the Interferometric Bidimensional Spectrometer and
the Imaging Magnetograph eXperiment. Velocities for different heights
of the solar atmosphere were determined by computing bisectors of the
Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet
analysis to study the intensity and velocity oscillations within
and around exploding granules. We also compared our observational
findings with predictions of numerical simulations. <BR /> Results:
Exploding granules have significantly longer lifetimes (10 to 15 min)
than regular granules. Exploding granules larger than 3.8″ form an
independent intergranular lane during their decay phase, while smaller
granules usually fade away or disappear into the intergranular area
(we find only one exception of a smaller exploding granule that also
forms an intergranular lane). For all exploding granules that form a new
intergranular downflow lane, we find a temporal height-dependent shift
with respect to the maximum of the downflow velocity. Our suggestion
that this results from a complex atmospheric structure within the newly
forming downflow lane is supported by the comparison with synthesised
profiles inferred from the simulations. We found an enhanced wavelet
power with periods between 120 s to 190 s seen in the intensity and
velocity oscillations of high photospheric or chromospheric spectral
lines in the region of the dark core of an exploding granule.
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Title: Modeling of 3d Atmospheres of Cool Stars with the Mancha Code
Authors: Perdomo, Andrea; Vitas, Nikola; Khomenko, Elena; Collados,
Manuel
2021csss.confE.129P Altcode:
The first results of the application of the MANCHA code to the case of
stars beyond the solar case are presented: hydrodynamical simulations
of stars of spectral type K0V and M0V compared with the solar case.
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Title: Influence of ambipolar and Hall effects on vorticity in
three-dimensional simulations of magneto-convection
Authors: Khomenko, E.; Collados, M.; Vitas, N.; González-Morales,
P. A.
2021RSPTA.37900176K Altcode: 2020arXiv200909753K
This paper presents the results of the analysis of three-dimensional
simulations of solar magneto-convection that include the joint action of
the ambipolar diffusion and the Hall effect. Three simulation runs are
compared: one including both ambipolar diffusion and the Hall effect;
one including only ambipolar diffusion and one without any of these
two effects. The magnetic field is amplified from initial field to
saturation level by the action of turbulent local dynamo. In each of
these cases, we study 2 h of simulated solar time after the local
dynamo reaches the saturation regime. We analyse the power spectra
of vorticity, of magnetic field fluctuations and of the different
components of the magnetic Poynting flux responsible for the transport
of vertical or horizontal perturbations. Our preliminary results show
that the ambipolar diffusion produces a strong reduction of vorticity
in the upper chromospheric layers and that it dissipates the vortical
perturbations converting them into thermal energy. The Hall effect
acts in the opposite way, strongly enhancing the vorticity. When the
Hall effect is included, the magnetic field in the simulations becomes,
on average, more vertical and long-lived flux tube-like structures are
produced. We trace a single magnetic structure to study its evolution
pattern and the magnetic field intensification, and their possible
relation to the Hall effect. <P />This article is part of the Theo
Murphy meeting issue `High-resolution wave dynamics in the lower
solar atmosphere'.
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Title: Joint action of Hall and ambipolar effects in 3D
magneto-convection simulations of the quiet Sun. I. Dissipation and
generation of waves
Authors: González-Morales, P. A.; Khomenko, E.; Vitas, N.; Collados,
M.
2020A&A...642A.220G Altcode: 2020arXiv200810429G
The partial ionization of the solar plasma causes several nonideal
effects such as the ambipolar diffusion, the Hall effect, and the
Biermann battery effect. Here we report on the first three-dimensional
realistic simulations of solar local dynamo where all three effects
were taken into account. The simulations started with a snapshot of
already saturated battery-seeded dynamo, where two new series were
developed: one with solely ambipolar diffusion and another one also
taking into account the Hall term in the generalized Ohm's law. The
simulations were then run for about 4 h of solar time to reach the
stationary regime and improve the statistics. In parallel, a purely
MHD dynamo simulation was also run for the same amount of time. The
simulations are compared in a statistical way. We consider the average
properties of simulation dynamics, the generation and dissipation
of compressible and incompressible waves, and the magnetic Poynting
flux. The results show that, with the inclusion of the ambipolar
diffusion, the amplitudes of the incompressible perturbations related
to Alfvén waves are reduced, and the Poynting flux is absorbed, with
a frequency dependence. The Hall effect causes the opposite action:
significant excess of incompressible perturbations is generated and an
excess of the Poynting flux is observed in the chromospheric layers. The
model with ambipolar diffusion shows, on average, sharper current
sheets and slightly more abundant fast magneto-acoustic shocks in the
chromosphere. The model with the Hall effect has higher temperatures at
the lower chromosphere and stronger and more vertical magnetic field
concentrations all over the chromosphere. The study of high-frequency
waves reveals that significant power of incompressible perturbations
is associated with areas with intense and more vertical magnetic
fields and larger temperatures. This behavior explains the large
Poynting fluxes in the simulations with the Hall effect and provides
confirmation as to the role of Alfvén waves in chromospheric heating
in internetwork regions, under the action of both Hall and ambipolar
effects. We find a positive correlation between the magnitude of the
ambipolar heating and the temperature increase at the same location
after a characteristic time of 10<SUP>2</SUP> s.
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Title: Local dynamo in stars beyond the Sun: Study for a K0V star
Authors: Perdomo García, A.; Vitas, N.; Khomenko, E.; Collados Vera,
M. A.
2020sea..confE.206P Altcode:
We present the first results of application of the MANCHA3D code (Felipe
2010; Khomenko et al. 2017, 2018) to a K0V cool star. Initially we run
the code solving purely hydrodynamic equations until the stationary
convection is reached. Then we produce the magnetic field generation
and amplification by Biermann's battery seed and local dynamo. We find
values around 100 Gauss for the amplified saturated magnetic field,
similar to those found in Khomenko et al. (2017) for the solar case.
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Title: Comparison of Parameters from Three-Dimensional
Magnetoconvection Simulations of the Solar Photosphere
Authors: Cubas Armas, M.; Fabbian, D.; Vitas, N.
2019ASPC..526..195C Altcode:
We present preliminary results from a comparison of parameters
derived from three-dimensional magnetohydrodynamic simulations of a
plage obtained with the MURaM and STAGGER codes. We first compared the
computed atmospheric physical parameters stored in the output temporal
snapshots. Subsequently, we used the same snapshots as input to perform
spectral synthesis calculations using the NICOLE code. We compared
the synthetic Fe I 630.1 nm and 630.2 nm spectral lines in terms
of central intensity, equivalent width, full width at half maximum,
as well as area and amplitude asymmetry of the Stokes V profile.
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Title: Three-dimensional simulations of solar magneto-convection
including effects of partial ionization
Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A.
2018A&A...618A..87K Altcode: 2018arXiv180701061K
In recent decades, REALISTIC three-dimensional
radiative-magnetohydrodynamic simulations have become the dominant
theoretical tool for understanding the complex interactions between the
plasma and magnetic field on the Sun. Most of such simulations are based
on approximations of magnetohydrodynamics, without directly considering
the consequences of the very low degree of ionization of the solar
plasma in the photosphere and bottom chromosphere. The presence of a
large amount of neutrals leads to a partial decoupling of the plasma and
magnetic field. As a consequence, a series of non-ideal effects, i.e.,
the ambipolar diffusion, Hall effect, and battery effect, arise. The
ambipolar effect is the dominant in the solar chromosphere. We
report on the first three-dimensional realistic simulations
of magneto-convection including ambipolar diffusion and battery
effects. The simulations are carried out using the newly developed
MANCHA3Dcode. Our results reveal that ambipolar diffusion causes
measurable effects on the amplitudes of waves excited by convection
in the simulations, on the absorption of Poynting flux and heating,
and on the formation of chromospheric structures. We provide a low
limit on the chromospheric temperature increase owing to the ambipolar
effect using the simulations with battery-excited dynamo fields. <P
/>The movies associated to Figs. 16 and 17 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833048/olm">https://www.aanda.org</A>
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Title: Numerical simulations of quiet Sun magnetic fields seeded by
the Biermann battery
Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A.
2017A&A...604A..66K Altcode: 2017arXiv170606037K
The magnetic fields of the quiet Sun cover at any time more than
90% of its surface and their magnetic energy budget is crucial to
explain the thermal structure of the solar atmosphere. One of the
possible origins of these fields is the action of the local dynamo
in the upper convection zone of the Sun. Existing simulations of the
local solar dynamo require an initial seed field and sufficiently
high spatial resolution in order to achieve the amplification of the
seed field to the observed values in the quiet Sun. Here we report
an alternative model of seeding based on the action of the Bierman
battery effect. This effect generates a magnetic field due to the
local imbalances in electron pressure in the partially ionized solar
plasma. We show that the battery effect self-consistently creates from
zero an initial seed field of a strength of the order of micro G, and
together with dynamo amplification allows the generation of quiet Sun
magnetic fields of a similar strength to those from solar observations.
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Title: DeepVel: Deep learning for the estimation of horizontal
velocities at the solar surface
Authors: Asensio Ramos, A.; Requerey, I. S.; Vitas, N.
2017A&A...604A..11A Altcode: 2017arXiv170305128A
Many phenomena taking place in the solar photosphere are controlled
by plasma motions. Although the line-of-sight component of the
velocity can be estimated using the Doppler effect, we do not have
direct spectroscopic access to the components that are perpendicular
to the line of sight. These components are typically estimated using
methods based on local correlation tracking. We have designed DeepVel,
an end-to-end deep neural network that produces an estimation of the
velocity at every single pixel, every time step, and at three different
heights in the atmosphere from just two consecutive continuum images. We
confront DeepVel with local correlation tracking, pointing out that they
give very similar results in the time and spatially averaged cases. We
use the network to study the evolution in height of the horizontal
velocity field in fragmenting granules, supporting the buoyancy-braking
mechanism for the formation of integranular lanes in these granules. We
also show that DeepVel can capture very small vortices, so that we can
potentially expand the scaling cascade of vortices to very small sizes
and durations. <P />The movie attached to Fig. 3 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730783/olm">http://www.aanda.org</A>
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Title: First Detection of Sign-reversed Linear Polarization from
the Forbidden [O I] 630.03 nm Line
Authors: de Wijn, A. G.; Socas-Navarro, H.; Vitas, N.
2017ApJ...836...29D Altcode: 2017arXiv170108793D
We report on the detection of linear polarization of the forbidden
[O I] 630.03 nm spectral line. The observations were carried out
in the broader context of the determination of the solar oxygen
abundance, an important problem in astrophysics that still remains
unresolved. We obtained spectro-polarimetric data of the forbidden
[O I] line at 630.03 nm as well as other neighboring permitted lines
with the Solar Optical Telescope of the Hinode satellite. A novel
averaging technique was used, yielding very high signal-to-noise ratios
in excess of 10<SUP>5</SUP>. We confirm that the linear polarization
is sign-reversed compared to permitted lines as a result of the line
being dominated by a magnetic dipole transition. Our observations
open a new window for solar oxygen abundance studies, offering an
alternative method to disentangle the Ni I blend from the [O I] line
at 630.03 nm that has the advantage of simple LTE formation physics.
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Title: Numerical simulations of the quiet-sun magnetic field:
Beyond MHD
Authors: Vitas, N.
2017psio.confE..37V Altcode:
No abstract at ADS
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Title: The role of partial ionization in solar chromospheric heating
Authors: Shelyag, S.; Khomenko, E.; Przybylski, D.; Vitas, N.; de
Vicente, A.
2016AGUFMSH21E2565S Altcode:
The most energetic part of the Sun, its interior, due to its plasma
parameters is hidden below the solar surface and invisible to the
observer. Nevertheless, the solar interior generates the energy and
provokes atmospheric magnetic activity. Despite great progress in
both observational and simulational methods, the mechanism of energy
transport from the solar convection zone into the upper atmosphere,
and the upper-atmospheric heating mechanism remain the main unresolved
problems in solar and stellar structure. In this presentation, we
analyse the role of non-ideal plasma effects and partial ionization in
the solar atmospheric energy transport and chromospheric heating. Using
numerical magneto-hydrodynamic modelling we create detailed models
of magnetic flux tubes and realistic simulations of the coupled
solar interior and atmosphere with different levels of magnetic
activity, which take into account the effects of partial ionisation
and ion-neutral interaction in the solar atmospheric plasma. We show
that compressible and incompressible oscillations in solar magnetic
fields, indeed, are able to provide sufficient energy to compensate
chromospheric radiative losses. Detailed radiative diagnostics of the
simulated models is carried out to create a link between the simulations
and observational data. This gives an opportunity to directly compare
the simulation results with modern solar observations.
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Title: How different are the Liège and Hamburg atlases of the
solar spectrum?
Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D.
2016A&A...590A.118D Altcode: 2016arXiv160403748D
Context. The high-fidelity solar spectral atlas prepared by
<A HREF="http://adsabs.harvard.edu/abs/1973apds.book.....D">
http://adsabs.harvard.edu/abs/1973apds.book.....D</A>
Delbouille et al. (Liège atlas, 1973) and the atlas by
<A HREF="http://adsabs.harvard.edu/abs/1999SoPh..184..421N">
http://adsabs.harvard.edu/abs/1999SoPh..184..421N</A> Neckel (Hamburg
atlas, 1999, Sol. Phys., 184, 421) are widely recognised as the most
important collection of reference spectra of the Sun at disc centre in
the visible wavelength range. The two datasets serve as fundamental
resources for many researchers, in particular for chemical abundance
analyses. But despite their similar published specifications (spectral
resolution and noise level), the shapes of the spectral lines in the
two atlases differ significantly and systematically. <BR /> Aims:
Knowledge of any instrumental degradations is imperative to fully
exploit the information content of spectroscopic data. We seek to
investigate the magnitude of these differences and explain the possible
sources. We provide the wavelength-dependent correction parameters that
need to be taken into account when the spectra are to be compared with
synthetic data, for instance. <BR /> Methods: A parametrically degraded
version of the Hamburg spectrum was fitted to the Liège spectrum. The
parameters of the model (wavelength shift, broadening, intensity
scaling, and intensity offset) represent the different characteristics
of the respective instruments, observational strategies, and data
processing. <BR /> Results: The wavelength scales of the Liège and
Hamburg atlases differ on average by 0.5 mÅ with a standard deviation
of ± 2 mÅ, except for a peculiar region around 5500 Å. The continuum
levels are offset by up to 18% below 5000 Å, but remain stably at a
0.8% difference towards the red. We find no evidence for spectral stray
light in the Liège spectrum. Its resolving power is almost independent
of wavelength but limited to about 216 000, which is between two to
six times lower than specified. When accounting for the degradations
determined in this work, the spectra of the two atlases agree to within
a few parts in 10<SUP>3</SUP>. <P />The fit parameters displayed in
Fig. 2 and derived data are only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A118">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A118</A>
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Title: VizieR Online Data Catalog: Differences of atlases of solar
spectrum (Doerr+, 2016)
Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D.
2016yCat..35900118D Altcode:
A parametrically degraded version of the Hamburg spectrum was fitted
to the Liege spectrum. The parameters of the model (wavelength shift,
broadening, intensity scaling, intensity offset) represent the different
characteristics of the respective instruments, observational strategies,
and data processing. The fits were carried out for all identified solar
lines in the line list provided by Pierce and Breckinridge (1973, The
Kitt-Peak Table of Solar Spectrum Wavelengths, Vol. Contribution No. 559
(Kitt Peak National Observatory); hereafter PB73) for an interval of
±15pm around the line cores. From an initial testrun we found that
the offset parameter (stray-light) is compatible with zero. The final
fits were carried out with only three free parameters. The results
from that run are provided in the file 'fitres.dat'. <P />A second
file (liegepar.dat) contains derived parameters (spectral resolving
power, position of the continuum) for the Liege spectrum that can be
used in studies that compare Liege data to other data (e.g. synthetic
spectra). <P />For instance, synthetic spectra have to be degraded to
match the spectral resolving power of the Liege spectrum for a valid
comparison. Our analysis showed that a Gaussian convolution kernel with
a full-width at half-maximum (FWHM) of lambda/R (lambda: wavelength; R:
spectral resolving power) is a very good approximation to the actual
instrumental profile of the Liege atlas. <P />Likewise, the continuum
of the Liege atlas at a particular wavelength can be matched to the
continuum of the Hamburg atlas by division with the parameter 'C'
from liegepar.dat. The correction parameters in liegepar.dat should
be interpolated to the desired wavelength range before being applied
in any data analysis. <P />We want to stress that the parameters
provided here result from the fitting-procedure as described in the
paper, with no further consistency checks or corrections applied. Some
fits are affected by nearby telluric blends. We recommend to apply an
outlier-rejection (e.g. a median filter) and/or smoothing before using
the data. The results need to be carefully checked. <P />(2 data files).
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Title: Fluid description of multi-component solar partially ionized
plasma
Authors: Khomenko, E.; Collados, M.; Díaz, A.; Vitas, N.
2014PhPl...21i2901K Altcode: 2014arXiv1408.1871K
We derive self-consistent formalism for the description of
multi-component partially ionized solar plasma, by means of the coupled
equations for the charged and neutral components for an arbitrary
number of chemical species, and the radiation field. All approximations
and assumptions are carefully considered. Generalized Ohm's law is
derived for the single-fluid and two-fluid formalism. Our approach is
analytical with some order-of-magnitude support calculations. After
general equations are developed, we particularize to some frequently
considered cases as for the interaction of matter and radiation.
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Title: Ellerman bombs: fallacies, fads, usage
Authors: Rutten, Robert J.; Vissers, Gregal J. M.; Rouppe van der
Voort, Luc H. M.; Sütterlin, Peter; Vitas, Nikola
2013JPhCS.440a2007R Altcode: 2013arXiv1304.1364R
Ellerman bombs are short-lived brightenings of the outer wings of Hα
that occur in active regions with much flux emergence. We point out
fads and fallacies in the extensive Ellerman bomb literature, discuss
their appearance in various spectral diagnostics, and advocate their
use as indicators of field reconfiguration in active-region topography
using AIA 1700 Å images.
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Title: On The Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, Bernard; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
2012AAS...22020701F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO<SUP>5</SUP>BOLD
code) to calculate Stokes profiles for the Fe I 6173 Å line
for snapshots of a sunspot, a plage area and an enhanced network
region. Stokes filtergrams are constructed for the 6 nominal HMI
wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B<SUB>0</SUB>(x,y,z) and vertical velocity V<SUB>0</SUB>(x,y,z)
in the simulations.
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Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
2012decs.confE.104F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD code)
to calculate Stokes profiles Fi(λ,x,y; i=I, V, Q, U) for the Fe I
6173 Å line for snapshots of a sunspot, a plage area and an enhanced
network region. Stokes filtergrams are constructed for the 6 nominal
HMI wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations.
---------------------------------------------------------
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.;
Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B.
2011sdmi.confE..74F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk in the
Fe I 6173 absorption line. We use the output of two high-resolution 3D,
time-dependent, radiative magneto-hydrodynamics simulations (one based
on the MURAM code, the other one on the COBOLD code) to calculate
Stokes profiles for the Fe I 6173 line for a snapshot of a plage
region and a snapshot of an enhanced network region. After spatially
degrading the Stokes profiles to HMI resolution, they are multiplied
by a representative set of HMI filter response functions and Stokes
filtergrams are constructed for the 6 nominal HMI wavelengths. The
magnetic field vector and line-of-sight Doppler velocities are
determined from these filtergrams using a simplified version of the HMI
magnetic field processing pipeline. Finally, the reconstructed magnetic
field is compared to the actual magnetic field in the simulation.
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Title: Fast horizontal flows in a quiet sun MHD simulation and their
spectroscopic signatures
Authors: Vitas, N.; Fischer, C. E.; Vögler, A.; Keller, C. U.
2011A&A...532A.110V Altcode:
Numerical simulations of solar surface convection have predicted
the existence of supersonic horizontal flows in the photospheric
granulation. Recently, the detection of such flows in data from the
Hinode satellite was reported. We study supersonic granular flows in
detail to understand their signatures in spectral lines and to test
the observational detection method used to identify these flows in
the Hinode observations. We perform time-dependent 3D radiative MHD
numerical simulations and synthesize the Fe i 6302 Å spectral lines at
the resolution of the Hinode data for different viewing angles covering
the center-limb variation. There is very large variation in the detailed
shape of the emergent line profiles depending on the viewing angle and
the particular flow properties and orientation. At the full simulation
resolution the supersonic flows can even produce distinct satellite
lines. After smearing to the Hinode resolution sufficient signature
of supersonic motion remains. Our analysis shows that the detection
method used to analyze the Hinode data is indeed applicable. However,
the detection is very sensitive to ad hoc parameter choices and can
also misidentify supersonic flows.
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Title: Line Shape Effects on Intensity Measurements of Solar Features:
Brightness Correction to SOHO MDI Continuum Images
Authors: Criscuoli, S.; Ermolli, I.; Del Moro, D.; Giorgi, F.;
Tritschler, A.; Uitenbroek, H.; Vitas, N.
2011ApJ...728...92C Altcode: 2010arXiv1012.2561C
Continuum intensity observations obtained with the Michelson
Doppler Imager (MDI) on board the SOHO mission provide long time
series of filtergrams that are ideal for studying the evolution of
large-scale phenomena in the solar atmosphere and their dependence on
solar activity. These filtergrams, however, are not taken in a pure
continuum spectral band, but are constructed from a proxy, namely a
combination of filtergrams sampling the Ni I 6768 Å line. We studied
the sensitivity of this continuum proxy to the shape of the nickel line
and to the degradation in the instrumental transmission profiles. We
compared continuum intensity measurements near the nickel line with
MDI proxy values in three sets of high-resolution spectro-polarimetric
data obtained with the Interferometric Bidimensional Spectrometer,
and in synthetic data, obtained from multi-dimensional simulations of
magneto-convection and one-dimensional atmosphere models. We found that
MDI continuum measurements require brightness corrections which depend
on magnetic field strength, temperature and, to a smaller extent, plasma
velocity. The correction ranges from 2% to 25% in sunspots, and is,
on average, less than 2% for other features. The brightness correction
also varies with position on the disk, with larger variations obtained
for sunspots, and smaller variations obtained for quiet Sun, faculae,
and micropores. Correction factors derived from observations agree
with those deduced from the numerical simulations when observational
effects are taken into account. Finally, we found that the investigated
potential uncertainties in the transmission characteristics of MDI
filters only slightly affect the brightness correction to proxy
measurements.
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Title: Observational signatures of the simulated solar photosphere
Authors: Vitas, N.
2011PhDT........83V Altcode:
Most of the visible light coming from the Sun originates in the solar
photosphere. Numerical simulations are a common tool to study the solar
atmosphere starting from the basic laws of physics and few additional
assumptions. This thesis presents results of three-dimensional radiative
magnetohydrodynamic simulations of the quiet-sun, plage and sunspot
umbra. These simulations are used to compute synthetic observations
that can be compared to the real observations. Each chapter of this
thesis tackles a different problem: the solar abundance of indium, the
formation of the only photospheric spectral line that shows activity
variation, the formation of the spectral lines in the horizontal
supersonic flows caused by convective motions, the center-to-limb
variation of the emergent continuum and the spectropolarimetric
diagnostic of the simulated sunspot umbra.
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Title: BAZA - Belgrade Astronomical Community Database
Authors: Atanackovic, O.; Vitas, N.; Arbutina, B.
2009POBeo..86..369A Altcode:
We present Belgrade astronomical community web site the purpose of
which is to provide basic information about students graduated from the
Department of Astronomy, Faculty of Mathematics, University of Belgrade,
as well as about our current students and friends worldwide, and to
help them communicating. BAZA (from Serbian: Beogradska Astronomska
ZAjednica) is available at http://alas.matf.bg.ac.yu/~astrobaza/
and http://astro.matf.bg.ac.yu/baza/.
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Title: Explanation of the activity sensitivity of Mn I 5394.7 Å
Authors: Vitas, N.; Viticchiè, B.; Rutten, R. J.; Vögler, A.
2009A&A...499..301V Altcode: 2008arXiv0811.3555V
There is a long-standing debate why the Mn i 5394.7 Å line in the
solar irradiance spectrum brightens more at higher activity than
other photospheric lines. The claim that this is caused by spectral
interlocking to chromospheric emission in the Mg ii h & k lines
is disputed. In this paper we settle this issue, using classical
one-dimensional modeling for demonstration and modern three-dimensional
MHD simulation for verification and analysis. The unusual sensitivity
of the Mn i 5394.7 Å line to solar activity is due to its excessive
hyperfine structure. This overrides the thermal and granular Doppler
smearing through which the other, narrower, photospheric lines lose
such sensitivity. We take the nearby Fe i 5395.2 Å line as example
of the latter, and analyze the formation of both lines in detail to
demonstrate and explain the granular Doppler brightening which affects
all narrow photospheric lines. Neither the chromosphere nor Mg ii h
& k emission play a role, nor is it correct to describe the activity
sensitivity of Mn i 5394.7 Å in terms of plage models with outward
increasing temperature contrast. The Mn i 5394.7 Å line represents a
proxy diagnostic of strong-field magnetic concentrations in the deep
solar photosphere comparable to the G band and the blue wing of Hα,
but not a better one than these. The Mn i lines are more promising as
diagnostics of weak fields in high-resolution Stokes polarimetry.
---------------------------------------------------------
Title: On the solar abundance of indium
Authors: Vitas, N.; Vince, I.; Lugaro, M.; Andriyenko, O.; Gošić,
M.; Rutten, R. J.
2008MNRAS.384..370V Altcode: 2008MNRAS.tmp...25V; 2007arXiv0711.2166V
The generally adopted value for the solar abundance of indium is over
six times higher than the meteoritic value. We address this discrepancy
through numerical synthesis of the 451.13-nm line on which all indium
abundance studies are based, both for the quiet Sun and the sunspot
umbra spectrum, employing standard atmosphere models and accounting
for hyperfine structure and Zeeman splitting in detail. The results,
as well as a re-appraisal of indium nucleosynthesis, suggest that
the solar indium abundance is close to the meteoritic value, and
that some unidentified ion line causes the 451.13-nm feature in the
quiet-Sun spectrum.
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Title: Is the Mn I 539.4 nm Variation with Activity Explained?
Authors: Vitas, N.; Vince, I.
2007ASPC..368..543V Altcode:
The photospheric Mn I 539.4 nm line in the solar spectrum shows unusual
variability with the solar cycle in that its depth decreases with
increasing activity. \citet{nv-2001A&A...369L..13D} claimed that
this phenomenon is due to interlocking between the chromospheric Mg II
h & k lines and an overlapping Mn I multiplet. In this contribution
we test this hypothesis by synthesizing Mn I 539.4 nm line including
these interlocking lines for a range of standard solar models and
then combining resulting profiles without and with interlocking to
emulate the full-disk profile variation. We find that the interlocking
gives only a minor contribution; the largest one comes from different
temperature stratifications in the photospheric layers of the various
models.
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Title: Formation of Neutral Manganese Lines Potentially Suitable
for Plasma Diagnostics
Authors: Vitas, N.; Danilović, S.; Atanacković-Vukmanović, O.;
Vince, Ištvan
2005ESASP.600E..73V Altcode: 2005dysu.confE..73V; 2005ESPM...11...73V
No abstract at ADS
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Title: Time Series Analysis of Long Term Full Disk Observations Of
The Mn I 539.4 nm Solar Line
Authors: Danilovic, S.; Vince, I.; Vitas, N.; Jovanovic, P.
2005SerAJ.170...79D Altcode:
The equivalent width and central depth data of Mn 539.4 nm solar
spectral line, observed in the period from 1979 to 1992 at Kitt
Peak Observatory, was analyzed in pursuit for periodic changes. As
the observations are highly unevenly sampled, test if the periods
really exist in the observed data was needed. Two different methods
for spectral analysis were applied to synthesized data sampled in the
same way as observations. Comparation of these results with results
obtained from the observed data showed that the parameters display
at least three periodic changes with the periods of: 11-years,
quasi-biannual and 27-days.
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Title: Heights of formation of Mn I spectral lines broadened by
hyperfine structure
Authors: Vitas, N.
2005MSAIS...7..164V Altcode:
This paper considers the influence of hyperfine broadening on heights
of formation of some Mn I spectral lines in Solar spectrum. The
comprehensive model atom of neutral manganese is constructed
with 64 bound energy levels and continuum and 161 bound-bound
transitions. Preliminary results of spectrum synthesis for this model
and atmospheric models for quiet Sun and plage are obtained by using
program MULTI. It is shown that hyperfine structure decreases the
height of formation and narrows down the line formation region.
---------------------------------------------------------
Title: NLTE Effects in formation of variable Mn I 539.4 nm line in
solar spectrum
Authors: Vitas, N.; Vince, I.
2005MmSAI..76.1064V Altcode:
On the basis of NLTE profile calculations obtained by employing the
radiative transfer code MULTI, the influence of the uncertainties
of various parameters important in the formation of the photospheric
Mn I 539.4 nm spectral line are discussed. Two cases are separately
considered: with and without energy transfer between Mg II and Mn
I. All calculations are performed with model atmospheres of Fontenla
et al. (1999).
---------------------------------------------------------
Title: The influence of hyperfine structure on the solar Mn I 543.25
nm line profile
Authors: Vitas, N.; Vince, O.; Vince, I.
2003KFNTS...4..142V Altcode:
The Mn I 543.25 nm spectral line has unusually broad line profile due to
its hyperfine structure. Here we present the results of LTE calculation
of the synthetical profile of the Mn I 543.25 nm line in the solar
spectrum. We used the Oxford total absorption oscillator strength
measurements as the first approximation for hyperfine structure. The
observed spectrum was taken from the High Resolution Solar Spectrum
Atlas. The relative intensities of six hyperfine components were varied
to obtain a good fit with the observed line profile.
---------------------------------------------------------
Title: The influence of hyperfine structure on some manganese line
profiles in the solar spectrum
Authors: Vitas, N.; Vince, I.
2003SerAJ.167...35V Altcode:
Taking into account effects of hyperfine structure, we calculated
the synthetic solar spectrum for wavelength intervals around nine
neutral manganese lines. To estimate values of hyperfine components
we used the Oxford total absorption oscillator strength measurements
(Booth et al. 1983). We compared observed profiles (Photometric Atlas
of the Solar Spectrum from 300 to 1000 nm (Delbouille et al. 1973))
of selected manganese lines with synthesized profiles in two cases:
when hyperfine structure is not and when it is taken into account. By
comparing the calculated with observed spectrum, we corrected the
total oscillator strengths of all nine selected manganese lines.
