Author name code: vitas ADS astronomy entries on 2022-09-14 author:"Vitas, Nikola" ------------------------------------------------------------------------ Title: Modeling the thermal conduction in the solar atmosphere with the code MANCHA3D Authors: Navarro, A.; Khomenko, E.; Modestov, M.; Vitas, N. Bibcode: 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.
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.
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.
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. Title: Newly formed downflow lanes in exploding granules in the solar photosphere Authors: Ellwarth, M.; Fischer, C. E.; Vitas, N.; Schmiz, S.; Schmidt, W. Bibcode: 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.
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.
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.
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. Title: Modeling of 3d Atmospheres of Cool Stars with the Mancha Code Authors: Perdomo, Andrea; Vitas, Nikola; Khomenko, Elena; Collados, Manuel Bibcode: 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. 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. Bibcode: 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.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. 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. Bibcode: 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 102 s. 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. Bibcode: 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. Title: Comparison of Parameters from Three-Dimensional Magnetoconvection Simulations of the Solar Photosphere Authors: Cubas Armas, M.; Fabbian, D.; Vitas, N. Bibcode: 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. Title: Three-dimensional simulations of solar magneto-convection including effects of partial ionization Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A. Bibcode: 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.

The movies associated to Figs. 16 and 17 are available at https://www.aanda.org Title: Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery Authors: Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A. Bibcode: 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. Title: DeepVel: Deep learning for the estimation of horizontal velocities at the solar surface Authors: Asensio Ramos, A.; Requerey, I. S.; Vitas, N. Bibcode: 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.

The movie attached to Fig. 3 is available at http://www.aanda.org 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. Bibcode: 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 105. 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. Title: Numerical simulations of the quiet-sun magnetic field: Beyond MHD Authors: Vitas, N. Bibcode: 2017psio.confE..37V Altcode: No abstract at ADS Title: The role of partial ionization in solar chromospheric heating Authors: Shelyag, S.; Khomenko, E.; Przybylski, D.; Vitas, N.; de Vicente, A. Bibcode: 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. Title: How different are the Liège and Hamburg atlases of the solar spectrum? Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D. Bibcode: 2016A&A...590A.118D Altcode: 2016arXiv160403748D Context. The high-fidelity solar spectral atlas prepared by http://adsabs.harvard.edu/abs/1973apds.book.....D Delbouille et al. (Liège atlas, 1973) and the atlas by http://adsabs.harvard.edu/abs/1999SoPh..184..421N 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.
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.
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.
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 103.

The fit parameters displayed in Fig. 2 and derived data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A118 Title: VizieR Online Data Catalog: Differences of atlases of solar spectrum (Doerr+, 2016) Authors: Doerr, H. -P.; Vitas, N.; Fabbian, D. Bibcode: 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'.

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).

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.

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.

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.

(2 data files). Title: Fluid description of multi-component solar partially ionized plasma Authors: Khomenko, E.; Collados, M.; Díaz, A.; Vitas, N. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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 CO5BOLD 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 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.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B. Bibcode: 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Observational signatures of the simulated solar photosphere Authors: Vitas, N. Bibcode: 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. Title: BAZA - Belgrade Astronomical Community Database Authors: Atanackovic, O.; Vitas, N.; Arbutina, B. Bibcode: 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/. Title: Explanation of the activity sensitivity of Mn I 5394.7 Å Authors: Vitas, N.; Viticchiè, B.; Rutten, R. J.; Vögler, A. Bibcode: 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. Bibcode: 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. Title: Is the Mn I 539.4 nm Variation with Activity Explained? Authors: Vitas, N.; Vince, I. Bibcode: 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. Title: Formation of Neutral Manganese Lines Potentially Suitable for Plasma Diagnostics Authors: Vitas, N.; Danilović, S.; Atanacković-Vukmanović, O.; Vince, Ištvan Bibcode: 2005ESASP.600E..73V Altcode: 2005dysu.confE..73V; 2005ESPM...11...73V No abstract at ADS 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. Bibcode: 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. Title: Heights of formation of Mn I spectral lines broadened by hyperfine structure Authors: Vitas, N. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.