Publications (30)43.48 Total impact
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Article: Heating of the Magnetized Solar Chromosphere by Partial Ionization Effects
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ABSTRACT: In this paper, we study the heating of the magnetized solar chromosphere induced by the large fraction of neutral atoms present in this layer. The presence of neutrals, together with the decrease with height of the collisional coupling, leads to deviations from the classical magnetohydrodynamic behavior of the chromospheric plasma. A relative net motion appears between the neutral and ionized components, usually referred to as ambipolar diffusion. The dissipation of currents in the chromosphere is enhanced by orders of magnitude due to the action of ambipolar diffusion, as compared with the standard ohmic diffusion. We propose that a significant amount of magnetic energy can be released to the chromosphere just by existing force-free 10-40 G magnetic fields there. As a consequence, we conclude that ambipolar diffusion is an important process that should be included in chromospheric heating models, as it has the potential to rapidly heat the chromosphere. We perform analytical estimations and numerical simulations to prove this idea.The Astrophysical Journal 02/2012; 747(2):87. · 6.02 Impact Factor -
Article: Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
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ABSTRACT: Full Stokes filter-polarimeters are key instruments for investigating the rapid evolution of magnetic structures on the solar surface. To this end, the image quality is routinely improved using a-posteriori image reconstruction methods. We analyze the robustness of circular polarization asymmetries to phase-diversity image reconstruction techniques. We use snapshots of magneto-hydrodynamical simulations carried out with different initial conditions to synthesize spectra of the magnetically sensitive Fe I line at 5250.2 A. We degrade the synthetic profiles spatially and spectrally to simulate observations with the IMaX full Stokes filter-polarimeter. We also simulate the focused/defocused pairs of images used by the phase-diversity algorithm for reconstruction and the polarimetric modulation scheme. We assume that standard optimization methods are able to infer the projection of the wavefront on the Zernike polynomials with 10% precision. We also consider the less favorable case of 25% precision. We obtain reconstructed monochromatic modulated images that are later demodulated and compared with the original maps. Although asymmetries are often difficult to define in the quiet Sun due to the complexity of the Stokes V profiles, we show how asymmetries are degraded with spatial and spectral smearing. The results indicate that, although image reconstruction techniques reduce the spatial smearing, they can modify the asymmetries of the profiles, mainly caused by the appearance of spatially-correlated noise.11/2011; -
Article: Magneto-acoustic wave energy from numerical simulations of an observed sunspot umbra
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ABSTRACT: We aim at reproducing the height dependence of sunspot wave signatures obtained from spectropolarimetric observations through 3D MHD numerical simulations. A magneto-static sunspot model based on the properties of the observed sunspot is constructed and perturbed at the photosphere introducing the fluctuations measured with the \SiI\ $\lambda$ 10827 \AA\ line. The results of the simulations are compared with the oscillations observed simultaneously at different heights from the \HeI\ $\lambda$ 10830 \AA\ line, the \CaIIH\ core and the \FeI\ blends in the wings of the \CaIIH\ line. The simulations show a remarkable agreement with the observations. They reproduce the velocity maps and power spectra at the formation heights of the observed lines, as well as the phase and amplification spectra between several pair of lines. We find that the stronger shocks at the chromosphere are accompanied with a delay between the observed signal and the simulated one at the corresponding height, indicating that shocks shift the formation height of the chromospheric lines to higher layers. Since the simulated wave propagation matches very well the properties of the observed one, we are able to use the numerical calculations to quantify the energy contribution of the magneto-acoustic waves to the chromospheric heating in sunspots. Our findings indicate that the energy supplied by these waves is too low to balance the chromospheric radiative losses. The energy contained at the formation height of the lowermost \SiI\ $\lambda$ 10827 \AA\ line in the form of slow magneto-acoustic waves is already insufficient to heat the higher layers, and the acoustic energy which reaches the chromosphere is around 3-9 times lower than the required amount of energy. The contribution of the magnetic energy is even lower.04/2011; -
Article: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX
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ABSTRACT: We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500 G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma motions. When granules evolve they squash the magnetic field lines together or pull them apart. Here, we report on the periodic deformation of the shapes of features in circular polarization observed at high resolution with SUNRISE. In particular, we note that the area of patches with a constant magnetic flux oscillates with time, which implies that the apparent magnetic field intensity oscillates in antiphase. The periods associated with this oscillatory pattern are compatible with the granular lifetime and change abruptly, which suggests that these oscillations might not correspond to characteristic oscillatory modes of magnetic structures, but to the forcing by granular motions. In one particular case, we find three patches around the same granule oscillating in phase, which means that the spatial coherence of these oscillations can reach 1600 km. Interestingly, the same kind of oscillatory phenomenon is also found in the upper photosphere.The Astrophysical Journal Letters 03/2011; 730(2):L37. · 5.53 Impact Factor -
Article: Where the Granular Flows Bend
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ABSTRACT: Based on IMaX/SUNRISE data, we report on a previously undetected phenomenon in solar granulation. We show that in a very narrow region separating granules and intergranular lanes, the spectral line width of the Fe I 5250.2 Å line becomes extremely small. We offer an explanation of this observation with the help of magneto-convection simulations. These regions with extremely small line widths correspond to the places where the granular flows bend from upflow in granules to downflow in intergranular lanes. We show that the resolution and image stability achieved by IMaX/SUNRISE are important requisites to detect this interesting phenomenon.The Astrophysical Journal Letters 10/2010; 723(2):L159. · 5.53 Impact Factor -
Article: Magneto-acoustic waves in sunspots from observations and numerical simulations
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ABSTRACT: We study the propagation of waves from the photosphere to the chromosphere of sunspots. From time series of cospatial Ca II H (including its line blends) intensity spectra and polarimetric spectra of Si I 1082.7 nm and He I 1083.0 nm we retrieve the line-of-sight velocity at several heights. The analysis of the phase difference and amplification spectra shows standing waves for frequencies below 4 mHz and propagating waves for higher frequencies, and allows us to infer the temperature and height where the lines are formed. Using these observational data, we have constructed a model of sunspot, and we have introduced the velocity measured with the photospheric Si I 1082.7 nm line as a driver. The numerically propagated wave pattern fits reasonably well with the observed using the lines formed at higher layers, and the simulations reproduce many of the observed features. The observed waves are slow MHD waves propagating longitudinally along field lines. Comment: proceedings of GONG 2010/SOHO 24 meeting, June 27 - July 2, 2010, Aix-en-Provence, France09/2010; -
Article: Multi-layer study of wave propagation in sunspots
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ABSTRACT: We analyze the propagation of waves in sunspots from the photosphere to the chromosphere using time series of co-spatial Ca II H intensity spectra (including its line blends) and polarimetric spectra of Si I 10827 and the He I 10830 multiplet. From the Doppler shifts of these lines we retrieve the variation of the velocity along the line-of-sight at several heights. Phase spectra are used to obtain the relation between the oscillatory signals. Our analysis reveals standing waves at frequencies lower than 4 mHz and a continuous propagation of waves at higher frequencies, which steepen into shocks in the chromosphere when approaching the formation height of the Ca II H core. The observed non-linearities are weaker in Ca II H than in He I lines. Our analysis suggests that the Ca II H core forms at a lower height than the He I 10830 line: a time delay of about 20 s is measured between the Doppler signal detected at both wavelengths. We fit a model of linear slow magnetoacoustic wave propagation in a stratified atmosphere with radiative losses according to Newton's cooling law to the phase spectra and derive the difference in the formation height of the spectral lines. We show that the linear model describes well the wave propagation up to the formation height of Ca II H, where non-linearities start to become very important. Comment: Accepted by The Astrophysical Journal08/2010; -
Article: The Sun at high resolution: first results from the Sunrise mission
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ABSTRACT: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the Sunrise data, which include a number of discoveries.Proceedings of the International Astronomical Union 07/2010; 6:226 - 232. -
Article: Magneto-acoustic waves in sunspots: first results from a new 3D nonlinear magnetohydrodynamic code
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ABSTRACT: Waves observed in the photosphere and chromosphere of sunspots show complex dynamics and spatial patterns. The interpretation of high-resolution sunspot wave observations requires modeling of three-dimensional non-linear wave propagation and mode transformation in the sunspot upper layers in realistic spot model atmospheres. Here we present the first results of such modeling. We have developed a 3D non-linear numerical code specially designed to calculate the response of magnetic structures in equilibrium to an arbitrary perturbation. The code solves the 3D nonlinear MHD equations for perturbations; it is stabilized by hyper-diffusivity terms and is fully parallelized. The robustness of the code is demonstrated by a number of standard tests. We analyze several simulations of a sunspot perturbed by pulses of different periods at subphotospheric level, from short periods, introduced for academic purposes, to longer and realistic periods of three and five minutes. We present a detailed description of the three-dimensional mode transformation in a non-trivial sunspot-like magnetic field configuration, including the conversion between fast and slow magneto-acoustic waves and the Alfv\'en wave, by calculation of the wave energy fluxes. Our main findings are the following: (1) the conversion from acoustic to the Alfv\'en mode is only observed if the the driving pulse is located out of the sunspot axis, but this conversion is energetically inefficient; (2) as a consequence of the cut-off effects and refraction of the fast magneto-acoustic mode, the energy of the evanescent waves with periods around 5 minutes remains almost completely below the level beta=1; (3) waves with frequencies above the cut-off propagate field-aligned to the chromosphere and their power becomes dominating over that of evanescent 5-minute oscillations, in agreement with observations.06/2010; -
Article: Mode transformation and frequency change with height in 3D numerical simulations of magneto-acoustic wave propagation in sunspots
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ABSTRACT: Three-dimensional numerical simulations of magnetoacoustic wave propagation are performed in a sunspot atmosphere with a computational domain covering from the photosphere to the chromosphere. The wave source, with properties resembling the solar spectrum, is located at different distances from the axis of the sunspot for each simulation. These results are compared with the theory of mode transformation and also with observational features. Simulations show that the dominant oscillation frequency in the chromosphere decreases with the radial distance from the sunspot axis. The energy flux of the different wave modes involved, including de Alfv\'en mode, is evaluated and discussed. Comment: Proceeding of the IV HELAS International Conference05/2010; -
Article: Numerical Simulation of Excitation and Propagation of Helioseismic MHD Waves in Magnetostatic Models of Sunspots
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ABSTRACT: We present comparison of numerical simulations of propagation of MHD waves,excited by subphotospheric perturbations, in two different ("deep" and "shallow") magnetostatic models of the sunspots. The "deep" sunspot model distorts both the shape of the wavefront and its amplitude stronger than the "shallow" model. For both sunspot models, the surface gravity waves (f-mode) are affected by the sunspots stronger than the acoustic p-modes. The wave amplitude inside the sunspot depends on the photospheric strength of the magnetic field and the distance of the source from the sunspot axis. For the source located at 9 Mm from the center of the sunspot, the wave amplitude increases when the wavefront passes through the central part of the sunspot. For the source distance of 12 Mm, the wave amplitude inside the sunspot is always smaller than outside. For the same source distance from the sunspot center but for the models with different strength of the magnetic field, the wave amplitude inside the sunspot increases with the strength of the magnetic field. The simulations show that unlike the case of the uniform inclined background magnetic field, the p- and f-mode waves are not spatially separated inside the sunspot where the magnetic field is strongly non-uniform. These properties have to be taken into account for interpretation of observations of MHD waves traveling through sunspot regions. Comment: 17 pages, 9 figures02/2010; -
Article: Sunspot seismic halos generated by fast MHD wave refraction
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ABSTRACT: We suggest an explanation for the high-frequency power excess surrounding active regions known as seismic halos. The idea is based on numerical simulations of magneto-acoustic waves propagation in sunspots. We propose that such an excess can be caused by the additional energy injected by fast mode waves refracted in the higher atmosphere due to the rapid increase of the Alfven speed. Our model qualitatively explains the magnitude of the halo and allows to make some predictions of its behavior that can be checked in future observations. Comment: Accepted by Astronomy and Astrophysics Letters05/2009; -
Article: The energy of waves in the photosphere and lower chromosphere: 1. Velocity statistics
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ABSTRACT: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE). We derived the mechanical wave energy as seen in line-core velocities to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model. We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume and mass energy densities. For comparison, we used the increase of internal energy necessary to lift a RE atmosphere to the HSRA temperature stratification. We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z~500 km is insufficient to maintain the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z~1000 km) can be traced back to the velocity variations of the lowermost forming spectral line considered (z~ 250 km). The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. Comment: 13 pages, 15 figures + 2 pages Appendix, 5 figures, submitted to A & A05/2009; -
Article: Theoretical Modeling of Propagation of Magnetoacoustic Waves in Magnetic Regions Below Sunspots
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ABSTRACT: We use two-dimensional numerical simulations and eikonal approximation to study properties of magnetohydrodynamic (MHD) waves traveling below the solar surface through the magnetic structure of sunspots. We consider a series of magnetostatic models of sunspots of different magnetic field strengths, from 10 Mm below the photosphere to the low chromosphere. The purpose of these studies is to quantify the effect of the magnetic field on local helioseismology measurements by modeling waves excited by subphotospheric sources. Time-distance propagation diagrams and wave travel times are calculated for models of various field strengths and compared to the nonmagnetic case. The results clearly indicate that the observed time-distance helioseismology signals in sunspot regions correspond to fast MHD waves. The slow MHD waves form a distinctly different pattern in the time-distance diagram, which has not been detected in observations. The numerical results are in good agreement with the solution in the short-wavelength (eikonal) approximation, providing its validation. The frequency dependence of the travel times is in good qualitative agreement with observations.The Astrophysical Journal 03/2009; 694(1):411. · 6.02 Impact Factor -
Article: On the Stokes V Amplitude Ratio as an Indicator of the Field Strength in the Solar Internetwork
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ABSTRACT: The results of the determination of magnetic field strength from weak polarimetric signals in solar internetwork regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes V amplitude ratio of the Fe I λλ15652-15648 lines is a good indicator of kG magnetic field concentrations, even for magnetic fields with a complex internal structure like those in MHD simulations. The Stokes V amplitude ratio of the Fe I λλ5247-5250 lines also shows a good correlation with magnetic field strength. However, in simulations with a flux level appropriate for the internetwork, it gives values corresponding to sub-kG fields. The reason is the rapid decrease of the field strength with height in kG magnetic field concentrations. These lines sample high regions of the atmosphere, where the field is already below kG levels. We also find that the Stokes V amplitude ratio of the Fe I λλ6301-6302 lines shows no correlation with the magnetic field strength. The reason lies in the large difference in the heights of formation of these two lines. The value of the magnetic field strength obtained from the Fe I λλ6301 and 6302 lines depends crucially on the treatment of gradients of the magnetic field, line-of-sight velocity, and temperature, even at a numerical spatial resolution of 20 km.The Astrophysical Journal 12/2008; 659(2):1726. · 6.02 Impact Factor -
Article: Numerical Modeling of Magnetohydrodynamic Wave Propagation and Refraction in Sunspots
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ABSTRACT: We present numerical simulations of magnetoacoustic wave propagation from the photosphere to the low chromosphere in a magnetic sunspot-like structure. A thick flux tube, with dimensions typical of a small sunspot, is perturbed by a vertical or horizontal velocity pulse at the photospheric level. The type of mode generated by the pulse depends on the ratio between the sound speed cS and the Alfvén speed vA, on the magnetic field inclination at the location of the driver, and on the shape of the pulse in the horizontal direction. Mode conversion is observed to occur in the region in which both characteristic speeds have similar values. The fast (magnetic) mode in the region cS < vA does not reach the chromosphere and reflects back to the photosphere at a somewhat higher layer than the cS = vA line. This behavior is due to wave refraction, caused primarily by the vertical and horizontal gradients of the Alfvén speed. The slow (acoustic) mode continues up to the chromosphere along the magnetic field lines with increasing amplitude. We show that this behavior is characteristic for waves in a wide range of periods generated at different distances from the sunspot axis. Since an important part of the energy of the pulse is returned back to the photosphere by the fast mode, the mechanism of energy transport from the photosphere to the chromosphere by waves in sunspots is rather ineffective.The Astrophysical Journal 12/2008; 653(1):739. · 6.02 Impact Factor -
Article: Theoretical modeling of propagation of magneto-acoustic waves in magnetic regions below sunspots
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ABSTRACT: We use 2D numerical simulations and eikonal approximation, to study properties of MHD waves traveling below the solar surface through the magnetic structure of sunspots. We consider a series of magnetostatic models of sunspots of different magnetic field strengths, from 10 Mm below the photosphere to the low chromosphere. The purpose of these studies is to quantify the effect of the magnetic field on local helioseismology measurements by modeling waves excited by sub-photospheric sources. Time-distance propagation diagrams and wave travel times are calculated for models of various field strength and compared to the non-magnetic case. The results clearly indicate that the observed time-distance helioseismology signals in sunspot regions correspond to fast MHD waves. The slow MHD waves form a distinctly different pattern in the time-distance diagram, which has not been detected in observations. The numerical results are in good agreement with the solution in the short-wavelength (eikonal) approximation, providing its validation. The frequency dependence of the travel times is in a good qualitative agreement with observations.09/2008; -
Article: Nonlinear Numerical Simulations of Magneto-Acoustic Wave Propagation in Small-Scale Flux Tubes
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ABSTRACT: We present results of nonlinear, two-dimensional, numerical simulations of magneto-acoustic wave propagation in the photosphere and chromosphere of small-scale flux tubes with internal structure. Waves with realistic periods of three to five minutes are studied, after horizontal and vertical oscillatory perturbations are applied to the equilibrium model. Spurious reflections of shock waves from the upper boundary are minimized by a special boundary condition. This has allowed us to increase the duration of the simulations and to make it long enough to perform a statistical analysis of oscillations. The simulations show that deep horizontal motions of the flux tube generate a slow (magnetic) mode and a surface mode. These modes are efficiently transformed into a slow (acoustic) mode in the v A<c S atmosphere. The slow (acoustic) mode propagates vertically along the field lines, forms shocks, and remains always within the flux tube. It might effectively deposit the energy of the driver into the chromosphere. When the driver oscillates with a high frequency, above the cutoff, nonlinear wave propagation occurs with the same dominant driver period at all heights. At low frequencies, below the cutoff, the dominant period of oscillations changes with height from that of the driver in the photosphere to its first harmonic (half period) in the chromosphere. Depending on the period and on the type of the driver, different shock patterns are observed.Solar Physics 08/2008; 251(1):589-611. · 2.78 Impact Factor -
Article: MHS sunspot model from deep sub-photospheric to chromospheric layers
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ABSTRACT: In order to understand the influence of magnetic fields on the propagation properties of waves, as derived from different local helioseismology techniques, forward modeling of waves is required. Such calculations need a model in magnetohydrostatic equilibrium as initial atmosphere to propagate oscillations through it. We provide a method to construct such a model in equilibrium for a wide range of parameters to be used for simulations of artificial helioseismologic data. The method combine the advantages of self-similar solutions and current-distributed models. A set of models is developed by numerical integration of magnetohydrostatic equations from the sub-photospheric to chromospheric layers. Comment: accepted by ApJ08/2008; -
Article: Observational signatures of numerically simulated MHD waves in small-scale fluxtubes
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ABSTRACT: We present some results obtained from the synthesis of Stokes profiles in small-scale flux tubes with propagating MHD waves. To that aim, realistic flux tubes showing internal structure have been excited with 5 min period drivers, allowing non-linear waves to propagate inside the magnetic structure. The observational signatures of these waves in Stokes profiles of several spectral lines that are commonly used in spectropolarimetric measurements are discussed.02/2008;
Top co-authors
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Institutions
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2008–2012
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Instituto de Astrofísica de Canarias
La Laguna, Canary Islands, Spain
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2006
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National Academy of Sciences of Ukraine
- Main Astronomical Observatory
Kharkiv, Kharkivs'ka Oblast', Ukraine
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