Author name code: svanda ADS astronomy entries on 2022-09-14 author:Svanda, Michal ------------------------------------------------------------------------ Title: The New Composite Solar Flare Index from Solar Cycle 17 to Cycle 24 (1937 - 2020) Authors: Velasco Herrera, Victor Manuel; Soon, Willie; Knoška, Štefan; Perez-Peraza, Jorge Alberto; Cionco, Rodolfo G.; Kudryavtsev, Sergey M.; Qiu, Shican; Connolly, Ronan; Connolly, Michael; Švanda, Michal; Acosta Jara, José; Gregori, Giovanni Pietro Bibcode: 2022SoPh..297..108V Altcode: The chromosphere is a highly dynamic outer plasma layer of the Sun. Its physical processes accounting for the variability are poorly understood. We reconstructed the solar chromospheric flare index (SFI) to study the solar chromospheric variability from 1937 to 2020. The new SFI database is a composite record of the Astronomical Institute Ondřejov Observatory of the Czech Academy of Sciences from 1937 - 1976 and the records of the Kandilli Observatory of Istanbul, Turkey from 1977 - 2020. The SFI records are available in daily, monthly, and yearly resolutions. We carried out the time-frequency analyses of the new 84-year long SFI records using the wavelet transform. We report the periodicities of 21.88 (Hale cycle), 10.94 (Schwabe cycle), 5.2 (quasi-quinquennial cycle), 3.5, 1.7, 1, 0.41 (or 149.7 days, Rieger cycle), 0.17 (62.1 days), 0.07 (25.9 days, solar rotational modulation) years. All these periodicities seem always present and persistent throughout the observational interval. Thus, we suggest that there is no reason to assume these solar periodicities are absent from other solar cycles. Time variations of the amplitude of each oscillation or periodicity were also studied using the inverse wavelet transform. We found that for the SFI the most active flare cycles over the record were Cycles 17, 19, and 21, while Cycles 20, 22, 23, and 24 were the weakest ones with Cycle 18 was intermediate in flare activity. This shows several differences to the equivalent relationships for solar activity implied by sunspot number records. Furthermore, this confirms that solar activity trends and variability in the chromosphere as captured by SFI are not necessarily the same as those of the Sun's photosphere, as implied by the sunspot number activity records, for instance. We have also introduced a new signal/noise wavelet coherence metric to analyze two different chromospheric indices available (i.e. the SFI and the disk-integrated chromospheric Ca II K activity indices) and to quantify the differences and similarities of the oscillations within the solar chromosphere. Our findings suggest the importance of carrying out additional co-analyses with other solar activity records to find physical inter-relations and connections between the different solar layers from the photosphere, the chromosphere to the corona. Title: Modelling of geomagnetically induced currents in the Czech transmission grid Authors: Švanda, Michal; Smičková, Anna; VýbošÅ¥oková, Tatiana Bibcode: 2021EP&S...73..229S Altcode: 2021arXiv211202438S We investigate the maximum expected magnitudes of the geomagnetically induced currents (GICs) in the Czech transmission power network. We compute a model utilising the Lehtinen-Pirjola method, considering the plane-wave model of the geoelectric field, and using the transmission network parameters kindly provided by the operator. We find that the maximum amplitudes expected in the nodes of the Czech transmission grid during the Halloween storm-like event are about 15 A. For the "extreme-storm" conditions with a 1-V/km geoelectric field, the expected maxima do not exceed 40 A. We speculate that the recently proven statistical correlation between the increased geomagnetic activity and anomaly rate in the power grid may be due to the repeated exposure of the devices to the low-amplitude GICs. Title: One-sided arc averaging geometries in time-distance local helioseismology Authors: Korda, David; Švanda, Michal; Roudier, Thierry Bibcode: 2021A&A...654A..84K Altcode: 2021arXiv210800872K Context. The study of solar oscillations (helioseismology) has been a very successful method of researching the Sun. Helioseismology teaches us about the structure and mean properties of the Sun. Together with mid-resolution data, the local properties were uncovered in quiet-Sun regions. However, magnetic fields affect the oscillations and prevent us from studying the properties of magnetically active regions with helioseismology.
Aims: We aim to create a new methodology to suppress the negative effects of magnetic fields on solar oscillations and measure plasma properties close to active regions.
Methods: The methodology consists of new averaging geometries, a non-linear approach to travel-time measurements, and a consistent inversion method that combines plasma flows and sound-speed perturbations.
Results: We constructed the one-sided arc averaging geometries and applied them to the non-linear approach of travel-time measurements. Using the one-sided arc travel times, we reconstructed the annulus travel times in a quiet-Sun region. We tested the methodology against the validated helioseismic inversion pipeline. We applied the new methodology for an inversion for surface horizontal flows in a region with a circular H-type sunspot. The inverted surface horizontal flows are comparable with the output of the coherent structure tracking, which is not strongly affected by the presence of the magnetic field. We show that the new methodology suppresses the negative effects of magnetic fields up to outer penumbra. We measure divergent flows with properties comparable to the moat flow.
Conclusions: The new methodology can teach us about the depth structure of active regions and physical conditions that contribute to the evolution of the active regions. Title: IRIS observations of chromospheric heating by acoustic waves in solar quiet and active regions Authors: Abbasvand, V.; Sobotka, M.; Švanda, M.; Heinzel, P.; Liu, W.; Mravcová, L. Bibcode: 2021A&A...648A..28A Altcode: 2021arXiv210208678A
Aims: To study the heating of solar chromospheric magnetic and nonmagnetic regions by acoustic and magnetoacoustic waves, the deposited acoustic-energy flux derived from observations of strong chromospheric lines is compared with the total integrated radiative losses.
Methods: A set of 23 quiet-Sun and weak-plage regions were observed in the Mg II k and h lines with the Interface Region Imaging Spectrograph (IRIS). The deposited acoustic-energy flux was derived from Doppler velocities observed at two different geometrical heights corresponding to the middle and upper chromosphere. A set of scaled nonlocal thermodynamic equilibrium 1D hydrostatic semi-empirical models - obtained by fitting synthetic to observed line profiles - was applied to compute the radiative losses. The characteristics of observed waves were studied by means of a wavelet analysis.
Results: Observed waves propagate upward at supersonic speed. In the quiet chromosphere, the deposited acoustic flux is sufficient to balance the radiative losses and maintain the semi-empirical temperatures in the layers under study. In the active-region chromosphere, the comparison shows that the contribution of acoustic-energy flux to the radiative losses is only 10−30%.
Conclusions: Acoustic and magnetoacoustic waves play an important role in the chromospheric heating, depositing a main part of their energy in the chromosphere. Acoustic waves compensate for a substantial fraction of the chromospheric radiative losses in quiet regions. In active regions, their contribution is too small to balance the radiative losses and the chromosphere has to be heated by other mechanisms. Title: Evolution and motions of magnetic fragments during the active region formation and decay: A statistical study Authors: Švanda, Michal; Sobotka, Michal; Mravcová, Lucia; Výbošťoková, Tatiana Bibcode: 2021A&A...647A.146S Altcode: 2021arXiv210202807S Context. The evolution of solar active regions is still not fully understood. The growth and decay of active regions have mostly been studied in case-by-case studies.
Aims: Instead of studying the evolution of active regions case by case, we performed a large-scale statistical study to find indications for the statistically most frequent scenario.
Methods: We studied a large sample of active regions recorded by the Helioseismic and Magnetic Imager instrument. The sample was split into two groups: forming (367 members) and decaying (679 members) active regions. We tracked individual dark features (i.e. those that are assumed to be intensity counterparts of magnetised fragments from small objects to proper sunspots) and followed their evolution. We investigated the statistically most often locations of fragment merging and splitting as well as their properties.
Results: Our results confirm that statistically, sunspots form by merging events of smaller fragments. The coalescence process is driven by turbulent diffusion in a process similar to random-walk, where supergranular flows seem to play an important role. The number of appearing fragments does not seem to significantly correlate with the number of sunspots formed. The formation seems to be consistent with the magnetic field accumulation. Statistically, the merging occurs most often between a large and a much smaller object. The decay of the active region seems to take place preferably by a process similar to the erosion. Title: Photospheric downflows observed with SDO/HMI, HINODE, and an MHD simulation Authors: Roudier, T.; Švanda, M.; Malherbe, J. M.; Ballot, J.; Korda, D.; Frank, Z. Bibcode: 2021A&A...647A.178R Altcode: 2021arXiv210303077R Downflows on the solar surface are suspected to play a major role in the dynamics of the convection zone, at least in its outer part. We investigate the existence of the long-lasting downflows whose effects influence the interior of the Sun but also the outer layers. We study the sets of Dopplergrams and magnetograms observed with Solar Dynamics Observatory and Hinode spacecrafts and an magnetohydrodynamic (MHD) simulation. All of the aligned sequences, which were corrected from the satellite motions and tracked with the differential rotation, were used to detect the long-lasting downflows in the quiet-Sun at the disc centre. To learn about the structure of the flows below the solar surface, the time-distance local helioseismology was used. The inspection of the 3D data cube (x, y, t) of the 24 h Doppler sequence allowed us to detect 13 persistent downflows. Their lifetimes lie in the range between 3.5 and 20 h with a sizes between 2″ and 3″ and speeds between −0.25 and −0.72 km s−1. These persistent downflows are always filled with the magnetic field with an amplitude of up to 600 Gauss. The helioseismic inversion allows us to describe the persistent downflows and compare them to the other (non-persistent) downflows in the field of view. The persistent downflows seem to penetrate much deeper and, in the case of a well-formed vortex, the vorticity keeps its integrity to the depth of about 5 Mm. In the MHD simulation, only sub-arcsecond downflows are detected with no evidence of a vortex comparable in size to observations at the surface of the Sun. The long temporal sequences from the space-borne allows us to show the existence of long-persistent downflows together with the magnetic field. They penetrate inside the Sun but are also connected with the anchoring of coronal loops in the photosphere, indicating a link between downflows and the coronal activity. A links suggests that EUV cyclones over the quiet Sun could be an effective way to heat the corona. Title: Plasma flows and sound-speed perturbations in the average supergranule Authors: Korda, David; Švanda, Michal Bibcode: 2021A&A...646A.184K Altcode: 2021arXiv210105731K Context. Supergranules create a peak in the spatial spectrum of photospheric velocity features. Even though they have some properties of convection cells, their origin is still being debated in the literature. The time-distance helioseismology constitutes a method that is suitable for investigating the deep structure of supergranules.
Aims: Our aim is to construct the model of the flows in the average supergranular cell using fully consistent time-distance inverse methodology.
Methods: We used the Multi-Channel Subtractive Optimally Localised Averaging inversion method with regularisation of the cross-talk. We combined the difference and the mean travel-time averaging geometries. We applied this methodology to travel-time maps averaged over more than 104 individual supergranular cells. These cells were detected automatically in travel-time maps computed for 64 quiet days around the disc centre. The ensemble averaging method allows us to significantly improve the signal-to-noise ratio and to obtain a clear picture of the flows in the average supergranule.
Results: We found near-surface divergent horizontal flows which quickly and monotonously weakened with depth; they became particularly weak at the depth of about 7 Mm, where they even apparently switched sign. The amplitude of the `reversed' flow was comparable to the background flows. The inverted vertical flows and sound-speed perturbations were spoiled by unknown systematic errors. To learn about the vertical component, we integrated the continuity equation from the surface. The derived estimates of the vertical flow depicted a sub-surface increase from about 5 m s-1 at the surface to about 35 m s-1 at the depth of about 3 Mm followed by a monotonous decrease to greater depths. The vertical flow remained positive (an upflow) and became indistinguishable from the background at the depth of about 15 Mm. We further detected a systematic flow in the longitudinal direction. The course of this systematic flow with depth agrees well with the model of the solar rotation in the sub-surface layers. Title: Observational study of chromospheric heating by acoustic waves Authors: Abbasvand, V.; Sobotka, M.; Švanda, M.; Heinzel, P.; García-Rivas, M.; Denker, C.; Balthasar, H.; Verma, M.; Kontogiannis, I.; Koza, J.; Korda, D.; Kuckein, C. Bibcode: 2020A&A...642A..52A Altcode: 2020arXiv200802688A
Aims: Our aim is to investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere. Observations in strong chromospheric lines are analyzed by comparing the deposited acoustic-energy flux with the total integrated radiative losses.
Methods: Quiet-Sun and weak-plage regions were observed in the Ca II 854.2 nm and Hα lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope on 2019 October 3 and in the Hα and Hβ lines with the echelle spectrograph attached to the Vacuum Tower Telescope on 2018 December 11 and 2019 June 6. The deposited acoustic energy flux at frequencies up to 20 mHz was derived from Doppler velocities observed in line centers and wings. Radiative losses were computed by means of a set of scaled non-local thermodynamic equilibrium 1D hydrostatic semi-empirical models obtained by fitting synthetic to observed line profiles.
Results: In the middle chromosphere (h = 1000-1400 km), the radiative losses can be fully balanced by the deposited acoustic energy flux in a quiet-Sun region. In the upper chromosphere (h > 1400 km), the deposited acoustic flux is small compared to the radiative losses in quiet as well as in plage regions. The crucial parameter determining the amount of deposited acoustic flux is the gas density at a given height.
Conclusions: The acoustic energy flux is efficiently deposited in the middle chromosphere, where the density of gas is sufficiently high. About 90% of the available acoustic energy flux in the quiet-Sun region is deposited in these layers, and thus it is a major contributor to the radiative losses of the middle chromosphere. In the upper chromosphere, the deposited acoustic flux is too low, so that other heating mechanisms have to act to balance the radiative cooling. Title: Exploiting Solar Visible-Range Observations by Inversion Techniques: From Flows in the Solar Subsurface to a Flaring Atmosphere Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová, Jana Bibcode: 2020rfma.book..349S Altcode: Observations of the Sun in the visible spectral range belong to standard measurements obtained by instruments both on the ground and in the space. Nowadays, both nearly continuous full-disc observations with medium resolution and dedicated campaigns of high spatial, spectral and/or temporal resolution constitute a holy grail for studies that can capture (both) the long- and short-term changes in the dynamics and energetics of the solar atmosphere. Observations of photospheric spectral lines allow us to estimate not only the intensity at small regions, but also various derived data products, such as the Doppler velocity and/or the components of the magnetic field vector. We show that these measurements contain not only direct information about the dynamics of solar plasmas at the surface of the Sun but also imprints of regions below and above it. Here, we discuss two examples: First, the local time-distance helioseismology as a tool for plasma dynamic diagnostics in the near subsurface and second, the determination of the solar atmosphere structure during flares. The methodology in both cases involves the technique of inverse modelling. Title: Immediate and delayed responses of power lines and transformers in the Czech electric power grid to geomagnetic storms Authors: Švanda, Michal; Mourenas, Didier; Žertová, Karla; Výbošt'oková, Tatiana Bibcode: 2020JSWSC..10...26S Altcode: 2020arXiv200514448S Eruptive events of solar activity often trigger abrupt variations of the geomagnetic field. Through the induction of electric currents, human infrastructures are also affected, namely the equipment of electric power transmission networks. It was shown in past studies that the rate of power-grid anomalies may increase after an exposure to strong geomagnetically induced currents. We search for a rapid response of devices in the Czech electric distribution grid to disturbed days of high geomagnetic activity. Such disturbed days are described either by the cumulative storm-time Dst or d(SYM-H)/dt low-latitude indices mainly influenced by ring current variations, by the cumulative AE high-latitude index measuring substorm-related auroral current variations, or by the cumulative ap mid-latitude index measuring both ring and auroral current variations. We use superposed epoch analysis to identify possible increases of anomaly rates during and after such disturbed days. We show that in the case of abundant series of anomalies on power lines, the anomaly rate increases significantly immediately (within 1 day) after the onset of geomagnetic storms. In the case of transformers, the increase of the anomaly rate is generally delayed by 2-3 days. We also find that transformers and some electric substations seem to be sensitive to a prolonged exposure to substorms, with a delayed increase of anomalies. Overall, we show that in the 5-day period following the commencement of geomagnetic activity there is an approximately 5-10% increase in the recorded anomalies in the Czech power grid and thus this fraction of anomalies is probably related to an exposure to GICs. Title: Evolution of photospheric flows under an erupting filament in the quiet-Sun region Authors: Wollmann, Jiří; Švanda, Michal; Korda, David; Roudier, Thierry Bibcode: 2020A&A...636A.102W Altcode: 2020arXiv200312515W Context. We studied the dynamics of the solar atmosphere in the region of a large quiet-Sun filament, which erupted on 21 October 2010. The filament eruption started at its northern end and disappeared from the Hα line-core filtergrams line within a few hours. The very fast motions of the northern leg were recorded in ultraviolet light by the Atmospheric Imaging Assembly (AIA) imager.
Aims: We aim to study a wide range of available datasets describing the dynamics of the solar atmosphere for five days around the filament eruption. This interval covers three days of the filament evolution, one day before the filament growth and one day after the eruption. We search for possible triggers that lead to the eruption of the filament.
Methods: The surface velocity field in the region of the filament were measured by means of time-distance helioseismology and coherent structure tracking. The apparent velocities in the higher atmosphere were estimated by tracking the features in the 30.4 nm AIA observations. To capture the evolution of the magnetic field, we extrapolated the photospheric line-of-sight magnetograms and also computed the decay index of the magnetic field.
Results: We found that photospheric velocity fields showed some peculiarities. Before the filament activation, we observed a temporal increase of the converging flows towards the filament's spine. In addition, the mean squared velocity increased temporarily before the activation and peaked just before it, followed by a steep decrease. We further see an increase in the average shear of the zonal flow component in the filament's region, followed by a steep decrease. The photospheric line-of-sight magnetic field shows a persistent increase of induction eastward from the filament spine. The decay index of the magnetic field at heights around 10 Mm shows a value larger than critical one at the connecting point of the northern filament end. The value of the decay index increases monotonically there until the filament activation. Then, it decreased sharply.

AIA30.4 nm movie is available at https://www.aanda.org Title: Chromospheric Heating by Acoustic Waves Compared to Radiative Cooling. II. Revised Grid of Models Authors: Abbasvand, Vahid; Sobotka, Michal; Heinzel, Petr; Švanda, Michal; Jurčák, Jan; del Moro, Dario; Berrilli, Francesco Bibcode: 2020ApJ...890...22A Altcode: 2020arXiv200103413A Acoustic and magnetoacoustic waves are considered to be possible agents of chromospheric heating. We present a comparison of deposited acoustic energy flux with total integrated radiative losses in the middle chromosphere of the quiet Sun and a weak plage. The comparison is based on a consistent set of high-resolution observations acquired by the Interferometric Bidimensional Spectrometer instrument in the Ca II 854.2 nm line. The deposited acoustic-flux energy is derived from Doppler velocities observed in the line core and a set of 1737 non-local thermodynamic equilibrium 1D hydrostatic semi-empirical models, which also provide the radiative losses. The models are obtained by scaling the temperature and column mass of five initial models by Vernazza et al. (1981; VAL) B-F to get the best fit of synthetic to observed profiles. We find that the deposited acoustic-flux energy in the quiet-Sun chromosphere balances 30%-50% of the energy released by radiation. In the plage, it contributes by 50%-60% in locations with vertical magnetic field and 70%-90% in regions where the magnetic field is inclined more than 50° to the solar surface normal. Title: Exploiting solar visible-range observations by inversion techniques: from flows in the solar subsurface to a flaring atmosphere Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová, Jana Bibcode: 2020arXiv200103874S Altcode: Observations of the Sun in the visible spectral range belong to standard measurements obtained by instruments both on the ground and in the space. Nowadays, both nearly continuous full-disc observations with medium resolution and dedicated campaigns of high spatial, spectral and/or temporal resolution constitute a holy grail for studies that can capture (both) the long- and short-term changes in the dynamics and energetics of the solar atmosphere. Observations of photospheric spectral lines allow us to estimate not only the intensity at small regions, but also various derived data products, such as the Doppler velocity and/or the components of the magnetic field vector. We show that these measurements contain not only direct information about the dynamics of solar plasmas at the surface of the Sun but also imprints of regions below and above it. Here, we discuss two examples: First, the local time-distance helioseismology as a tool for plasma dynamic diagnostics in the near subsurface and second, the determination of the solar atmosphere structure during flares. The methodology in both cases involves the technique of inverse modelling. Title: Comparison of time-distance inversion methods applied to SDO/HMI Dopplergrams Authors: Korda, David; Švanda, Michal; Zhao, Junwei Bibcode: 2019A&A...629A..55K Altcode: 2019arXiv190803950K Context. The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) satellite has been observing the Sun since 2010. The uninterrupted series of Dopplergrams are ideal for studying the dynamics of the upper solar convection zone. Within the Joint Science Operations Center (JSOC) the time-distance inversions for flows and sound-speed perturbations were introduced. The automatic pipeline has produced flow and sound-speed maps every 8 h. We verify the results of JSOC inversions by comparing the data products to equivalent results from inverse modelling obtained by an independent inversion pipeline.
Aims: We compared the results from the JSOC pipeline for horizontal flow components and the perturbations of the speed of sound at set of depths with equivalent results from an independently implemented pipeline using a different time-distance inversion scheme. Our inversion pipeline allows inversion for all quantities at once while allowing minimisation of the crosstalk between them. This gives us an opportunity to discuss the possible biases present in the JSOC data products.
Methods: For the tests we used the subtractive optimally localised averaging (SOLA) method with a minimisation of the cross-talk. We compared three test inversions for each quantity at each target depth. At first, we used the JSOC setup to reproduce the JSOC results. Subsequently, we used the extended pipeline to improve these results by incorporating more independent travel-time measurements but keeping the JSOC-indicated localisation in the Sun. Finally, we inverted for flow components and sound-speed perturbations using a localisation kernel with properties advertised in the JSOC metadata.
Results: We successfully reproduced the horizontal flow components. The sound-speed perturbations are strongly affected by the high level of the cross-talk in JSOC products. This leads to larger amplitudes in the inversions for the sound-speed perturbations. Different results were obtained when a target function localised around the target depth was used. This is a consequence of non-localised JSOC averaging kernels. We add that our methodology also allows inversion for the vertical flow. Title: A New Look into Putative Duplicity and Pulsations of the Be Star β CMi Authors: Harmanec, P.; Švanda, M.; Korčáková, D.; Chini, R.; Nasseri, A.; Yang, S.; Božić, H.; Šlechta, M.; Vanzi, L. Bibcode: 2019ApJ...875...13H Altcode: 2019arXiv190207450H Bright Be star β CMi has been identified as a nonradial pulsator on the basis of space photometry with the Microvariability and Oscillations of Stars (MOST) satellite and also as a single-line spectroscopic binary with a period of 170.ͩ4. The purpose of this study is to re-examine both these findings using numerous electronic spectra from the Dominion Astrophysical Observatory, Ondřejov Observatory, Universitätssterwarte Bochum, archival electronic spectra from several observatories, as well as the original MOST satellite photometry. We measured the radial velocity of the outer wings of the double Hα emission in all spectra at our disposal, and were not able to confirm significant radial-velocity changes. We also discuss the problems related to the detection of very small radial-velocity changes and conclude that while it is still possible that the star is a spectroscopic binary, there is currently no convincing proof of it from the radial-velocity measurements. Wavelet analysis of the MOST photometry shows that there is only one persistent (and perhaps slightly variable) periodicity of 0.ͩ617 of the light variations, with a double-wave light curve; all other short periods having only transient character. Our suggestion that this dominant period is the star’s rotational period agrees with the estimated stellar radius, projected rotational velocity, and with the orbital inclination derived by two teams of investigators. New spectral observations obtained in the whole-night series would be needed to find out whether some possibly real, very small radial-velocity changes cannot, in fact, be due to rapid line-profile changes.

Based on spectral observations obtained at the Dominion Astrophysical Observatory, NRC Herzberg, Programs in Astronomy and Astrophysics, National Research Council of Canada, Ondřejov Observatory and Universitätssternwarte Bochum, and on photometry from the Canadian MOST satellite and UBV observations from the Hvar Observatory. Title: Combined helioseismic inversions for 3D vector flows and sound-speed perturbations Authors: Korda, David; Švanda, Michal Bibcode: 2019A&A...622A.163K Altcode: 2019arXiv190101293K Context. Time-distance helioseismology is the method of the study of the propagation of waves through the solar interior via the travel times of those waves. The travel times of wave packets contain information about the conditions in the interior integrated along the propagation path of the wave. The travel times are sensitive to perturbations of a variety of quantities. The usual task is to invert for the vector of plasma flows or the sound-speed perturbations separately. The separate inversions may be polluted by systematic bias, for instance, originating in the leakage of vector flows into the sound-speed perturbations and vice versa (called a cross-talk). Information about the cross-talk is necessary for a proper interpretation of results.
Aims: We introduce an improved methodology of the time-distance helioseismology which allows us to invert for a full 3D vector of plasma flows and the sound-speed perturbations at once. Using this methodology one can also derive the mean value of the vertical component of plasma flows and the cross-talk between the plasma flows and the sound-speed perturbations.
Methods: We used the Subtractive Optimally Localised Averaging method with a minimisation of the cross-talk as a tool for inverse modelling. In the forward model, we use Born approximation travel-time sensitivity kernels with the Model S as a background. The methodology was validated using forward-modelled travel times with both mean and difference point-to-annulus averaging geometries applied to a snapshot of fully self-consistent simulation of the convection.
Results: We tested the methodology on synthetic data. We demonstrate that we are able to recover flows and sound-speed perturbations in the near-surface layers. We have taken the advantage of the sensitivity of our methodology to entire vertical velocity, and not only to its variations as in other available methodologies. The cross-talk from both the vertical flow component and the sound-speed perturbation has only a negligible effect for inversions for the horizontal flow components. Furthermore, this cross-talk can be minimised if needed. The inversions for the vertical component of the vector flows or for the sound-speed perturbations are affected by the cross-talk from the horizontal components, which needs to be minimised in order to provide valid results. It seems that there is a nearly constant cross-talk between the vertical component of the vector flows and the sound-speed perturbations. Title: Heating of the solar photosphere during a white-light flare Authors: Jurčák, Jan; Kašparová, Jana; Švanda, Michal; Kleint, Lucia Bibcode: 2018A&A...620A.183J Altcode: 2018arXiv181107794J Context. The Fe I lines observed by the Hinode/SOT spectropolarimeter were always seen in absorption, apart from the extreme solar limb. Here we analyse a unique dataset capturing these lines in emission during a solar white-light flare.
Aims: We analyse the temperature stratification in the solar photosphere during a white-light flare and compare it with the post-white-light flare state.
Methods: We used two scans of the Hinode/SOT spectropolarimeter to infer, by means of the LTE inversion code Stokes Inversion based on Response function (SIR), the physical properties in the solar photosphere during and after a white-light flare. The resulting model atmospheres are compared and the changes are related to the white-light flare.
Results: We show that the analysed white-light flare continuum brightening is probably not caused by the temperature increase at the formation height of the photospheric continuum. However, the photosphere is heated by the flare approximately down to log τ = -0.5 and this results in emission profiles of the observed Fe I lines. From the comparison with the post-white-light flare state of the atmosphere, we estimate that the major contribution to the increase in the continuum intensity originates in the heated chromosphere. Title: Understanding the HMI Pseudocontinuum in White-light Solar Flares Authors: Švanda, Michal; Jurčák, Jan; Kašparová, Jana; Kleint, Lucia Bibcode: 2018ApJ...860..144S Altcode: 2018arXiv180503369S We analyze observations of the X9.3 solar flare (SOL2017-09-06T11:53) observed by SDO/HMI and Hinode/Solar Optical Telescope. Our aim is to learn about the nature of the HMI pseudocontinuum I c used as a proxy for the white-light continuum. From model atmospheres retrieved by an inversion code applied to the Stokes profiles observed by the Hinode satellite, we synthesize profiles of the Fe I 617.3 nm line and compare them to HMI observations. Based on a pixel-by-pixel comparison, we show that the value of I c represents the continuum level well in quiet-Sun regions only. In magnetized regions, it suffers from a simplistic algorithm that is applied to a complex line shape. During this flare, both instruments also registered emission profiles in the flare ribbons. Such emission profiles are poorly represented by the six spectral points of HMI and the MDI-like algorithm does not account for emission profiles in general; thus, the derived pseudocontinuum intensity does not approximate the continuum value properly. Title: Large-scale photospheric motions determined from granule tracking and helioseismology from SDO/HMI data Authors: Roudier, Th.; Švanda, M.; Ballot, J.; Malherbe, J. M.; Rieutord, M. Bibcode: 2018A&A...611A..92R Altcode: 2017arXiv171205255R Context. Large-scale flows in the Sun play an important role in the dynamo process linked to the solar cycle. The important large-scale flows are the differential rotation and the meridional circulation with an amplitude of km s-1 and few m s-1, respectively. These flows also have a cycle-related components, namely the torsional oscillations. Aim. Our attempt is to determine large-scale plasma flows on the solar surface by deriving horizontal flow velocities using the techniques of solar granule tracking, dopplergrams, and time-distance helioseismology.
Methods: Coherent structure tracking (CST) and time-distance helioseismology were used to investigate the solar differential rotation and meridional circulation at the solar surface on a 30-day HMI/SDO sequence. The influence of a large sunspot on these large-scale flows with a specific 7-day HMI/SDO sequence has been also studied.
Results: The large-scale flows measured by the CST on the solar surface and the same flow determined from the same data with the helioseismology in the first 1 Mm below the surface are in good agreement in amplitude and direction. The torsional waves are also located at the same latitudes with amplitude of the same order. We are able to measure the meridional circulation correctly using the CST method with only 3 days of data and after averaging between ± 15° in longitude.
Conclusions: We conclude that the combination of CST and Doppler velocities allows us to detect properly the differential solar rotation and also smaller amplitude flows such as the meridional circulation and torsional waves. The results of our methods are in good agreement with helioseismic measurements. Title: Differential rotation in magnetic chemically peculiar stars Authors: Mikulášek, Z.; Krtička, J.; Paunzen, E.; Švanda, M.; Hummerich, S.; Bernhard, K.; Jagelka, M.; Janík, J.; Henry, G. W.; Shultz, M. E. Bibcode: 2018CoSka..48..203M Altcode: Magnetic chemically peculiar (mCP) stars constitute about 10% of upper-main-sequence stars and are characterized by strong magnetic fields and abnormal photospheric abundances of some chemical elements. Most of them exhibit strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by a rigidly rotating main-sequence star with persistent surface structures and a stable global magnetic field. Long-term observations of the phase curves of these variations enable us to investigate possible surface differential rotation with unprecedented accuracy and reliability. The analysis of the phase curves in the best-observed mCP stars indicates that the location and the contrast of photometric and spectroscopic spots as well as the geometry of the magnetic field remain constant for at least many decades. The strict periodicity of mCP variables supports the concept that the outer layers of upper-main-sequence stars do not rotate differentially. However, there is a small, inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades. The period oscillations may reflect real changes in the angular velocity of outer layers of the stars which are anchored by their global magnetic fields. In CU Vir, V901 Ori, and perhaps BS Cir, the rotational period variation indicates the presence of vertical differential rotation; however, its exact nature has remained elusive until now. The incidence of mCP stars with variable rotational periods is currently investigated using a sample of fifty newly identified Kepler mCP stars. Title: Testing the Wavelet Analysis on the Evolution of the Polaris Pulsation Period using the SMEI Photometry Authors: Švanda, Michal; Harmanec, Petr Bibcode: 2017RNAAS...1...39S Altcode: 2017RNAAS...1a..39S No abstract at ADS Title: Automatic detection of white-light flare kernels in SDO/HMI intensitygrams Authors: Mravcová, Lucia; Švanda, Michal Bibcode: 2017NewA...57...14M Altcode: 2017arXiv170600988M Solar flares with a broadband emission in the white-light range of the electromagnetic spectrum belong to most enigmatic phenomena on the Sun. The origin of the white-light emission is not entirely understood. We aim to systematically study the visible-light emission connected to solar flares in SDO/HMI observations. We developed a code for automatic detection of kernels of flares with HMI intensity brightenings and study properties of detected candidates. The code was tuned and tested and with a little effort, it could be applied to any suitable data set. By studying a few flare examples, we found indication that HMI intensity brightening might be an artefact of the simplified procedure used to compute HMI observables. Title: Estimate of the regularly gridded 3D vector flow field from a set of tomographic maps Authors: Švanda, Michal; Kozoň, Marek Bibcode: 2017A&A...600A.117S Altcode: 2016arXiv161208275S Time-distance inversions usually provide tomographic maps of the interesting plasma properties (we focus on flows) at various depths. These maps, however, do not correspond directly to the flow field, but rather to the true flow field smoothed by the averaging kernels. We introduce a method to derive a regularly gridded estimate of the true velocity field from a set of tomographic maps. We mainly aim to reconstruct the flow on a uniform grid in the vertical domain. We derive the algorithm, implement it and validate using synthetic data. The use of the synthetic data allows us to investigate the influence of random noise and to develop the methodology to deal with it properly. Title: Differential rotation, flares and coronae in A to M stars Authors: Balona, L. A.; Švanda, M.; Karlický, M. Bibcode: 2016MNRAS.463.1740B Altcode: 2016MNRAS.tmp.1226B Kepler data are used to investigate flares in stars of all spectral types. There is a strong tendency across all spectral types for the most energetic flares to occur among the most rapidly rotating stars. Differential rotation could conceivably play an important role in enhancing flare energies. This idea was investigated, but no correlation could be found between rotational shear and the incidence of flares. Inspection of Kepler light curves shows that rotational modulation is very common over the whole spectral type range. Using the rotational light amplitude, the size distribution of star-spots was investigated. Our analysis suggests that stars with detectable flares have spots significantly larger than non-flare stars, indicating that flare energies are correlated with the size of the active region. Further evidence of the existence of spots on A stars is shown by the correlation between the photometric period and the projected rotational velocity. The existence of spots indicates the presence of magnetic fields, but the fact that A stars lack coronae implies that surface convection is a necessary condition for the formation of the corona. Title: Flares on A-type Stars: Evidence for Heating of Solar Corona by Nanoflares? Authors: Švanda, Michal; Karlický, Marian Bibcode: 2016ApJ...831....9S Altcode: 2016arXiv160803494S We analyzed the occurrence rates of flares on stars of spectral types K, G, F, and A, observed by Kepler. We found that the histogram of occurrence frequencies of stellar flares is systematically shifted toward a high-energy tail for A-type stars compared to stars of cooler spectral types. We extrapolated the fitted power laws toward flares with smaller energies (nanoflares) and made estimates for total energy flux to stellar atmospheres by flares. We found that, for A-type stars, the total energy flux density was at least four-times smaller than for G stars. We speculate that this deficit in energy supply may explain the lack of hot coronae on A-type stars. Our results indicate the importance of nanoflares for heating and formation of the solar corona. Title: Chromospheric Heating by Acoustic Waves Compared to Radiative Cooling Authors: Sobotka, M.; Heinzel, P.; Švanda, M.; Jurčák, J.; del Moro, D.; Berrilli, F. Bibcode: 2016ApJ...826...49S Altcode: 2016arXiv160504794S Acoustic and magnetoacoustic waves are among the possible candidate mechanisms that heat the upper layers of the solar atmosphere. A weak chromospheric plage near the large solar pore NOAA 11005 was observed on 2008 October 15, in the Fe I 617.3 nm and Ca II 853.2 nm lines of the Interferometric Bidimemsional Spectrometer attached to the Dunn Solar Telescope. In analyzing the Ca II observations (with spatial and temporal resolutions of 0.″4 and 52 s) the energy deposited by acoustic waves is compared to that released by radiative losses. The deposited acoustic flux is estimated from the power spectra of Doppler oscillations measured in the Ca II line core. The radiative losses are calculated using a grid of seven one-dimensional hydrostatic semi-empirical model atmospheres. The comparison shows that the spatial correlation of the maps of radiative losses and acoustic flux is 72%. In a quiet chromosphere, the contribution of acoustic energy flux to radiative losses is small, only about 15%. In active areas with a photospheric magnetic-field strength between 300 and 1300 G and an inclination of 20°-60°, the contribution increases from 23% (chromospheric network) to 54% (a plage). However, these values have to be considered as lower limits and it might be possible that the acoustic energy flux is the main contributor to the heating of bright chromospheric network and plages. Title: Polar cap magnetic field reversals during solar grand minima: could pores play a role? Authors: Švanda, Michal; Brun, Allan Sacha; Roudier, Thierry; Jouve, Laurène Bibcode: 2016A&A...586A.123S Altcode: 2015arXiv151106894S We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40-100 Mm from the closest active region systematically have the correct polarity of the magnetic field to contribute to the polar cap reversal. These pores can be found predominantly in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, which is akin to a weak magnetic (Babcock-Leighton-like) source term, could still be operating but was missed by the contemporary observers because of the limited resolving power of their telescopes. Title: Issues with time-distance inversions for supergranular flows Authors: Švanda, Michal Bibcode: 2015A&A...575A.122S Altcode: 2015arXiv150104160S
Aims: Recent studies have shown that time-distance inversions for flows start to be dominated by a random noise at a depth of only a few Mm. It was proposed that the ensemble averaging might be a solution for learning about the structure of the convective flows, e.g. about the depth structure of supergranulation.
Methods: Time-distance inversion is applied to the statistical sample of ∼ 104 supergranules, which allows the inversion cost function to be regularised weakly about the random-noise term and thus provides a much better localisation in space. We compare these inversions at four depths (1.9, 2.9, 4.3, and 6.2 Mm) when using different spatio-temporal filtering schemes in order to gain confidence about these inferences.
Results: The flows inferred by using different spatio-temporal filtering schemes are different (even by the sign) even though the formal averaging kernels and the random-noise levels are very similar. The inverted flows changes its sign several times with depth. I suggest that this is due to the inaccuracies in the forward problem that are possibly amplified by the inversion. It is also possible that other time-distance inversions are affected by this. Title: The PLATO 2.0 mission Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz, W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon, L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse, M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.; Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.; Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.; Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.; Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.; Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.; Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.; Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.; Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.; Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer, R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille, T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt, M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels, A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker, S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko, M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.; Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal, P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur, S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro, M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.; Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack, L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics, P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.; Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.; Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne, A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio, A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.; Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó, Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair, S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.; Werner, S. C.; Wheatley, P. J.; Zwintz, K. Bibcode: 2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence) providing a wide field-of-view (2232 deg 2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmosphere. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science. Title: Moat Flow System around Sunspots in Shallow Subsurface Layers Authors: Švanda, Michal; Sobotka, Michal; Bárta, Tomáš Bibcode: 2014ApJ...790..135S Altcode: 2014arXiv1406.2482S We investigate the subsurface moat flow system around McIntosh H-type symmetrical sunspots and compare it to the flow system within supergranular cells. Representatives of both types of flows are constructed by means of the statistical averaging of flow maps obtained by time-distance helioseismic inversions. We find that moat flows around H-type sunspots replace supergranular flows but there are two principal differences between the two phenomena: the moat flow is asymmetrical, probably due to the proper motion of sunspots with respect to the local frame of rest, while the flow in the supergranular cell is highly symmetrical. Furthermore, the whole moat is a downflow region, while the supergranule contains the upflow in the center, which turns into the downflow at about 60% of the cell radius from its center. We estimate that the mass downflow rate in the moat region is at least two times larger than the mass circulation rate within the supergranular cell. Title: Structure and evolution of solar supergranulation using SDO/HMI data Authors: Roudier, Th.; Švanda, M.; Rieutord, , M.; Malherbe, J. M.; Burston, R.; Gizon, L. Bibcode: 2014A&A...567A.138R Altcode: 2014arXiv1407.0196R Context. Studying the motions on the solar surface is fundamental for understanding how turbulent convection transports energy and how magnetic fields are distributed across the solar surface.
Aims: From horizontal velocity measurements all over the visible disc of the Sun and using data from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI), we investigate the structure and evolution of solar supergranulation.
Methods: Horizontal velocity fields were measured by following the proper motions of solar granules using a newly developed version of the coherent structure tracking code. With this tool, maps of horizontal divergence were computed. We then segmented and identified supergranular cells and followed their histories by using spatio-temporal labelling. With this data set we derived the fundamental properties of supergranulation, including their motion.
Results: We find values of the fundamental parameters of supergranulation similar to previous studies: a mean lifetime of 1.5 days and a mean diameter of 25 Mm. The tracking of individual supergranular cells reveals the solar differential rotation and a poleward circulation trend of the meridional flow. The shape of the derived differential rotation and meridional flow does not depend on the cell size. If there is a background magnetic field, the diverging flows in supergranules are weaker.
Conclusions: This study confirms that supergranules are suitable tracers that may be used to investigate the large-scale flows of the solar convection as long as they are detectable enough on the surface. Title: An Estimate of Chromospheric Heating by Acoustic Waves Authors: Sobotka, M.; Švanda, M.; Jurčak, J.; Heinzel, P.; Del Moro, D.; Berrilli, F. Bibcode: 2014CEAB...38...53S Altcode: Several mechanisms may heat the solar chromosphere: acoustic waves, magnetoacoustic waves (slow, fast, and Alfvén waves), and small-scale magnetic reconnections. Based on observations in the Ca II 854.2 nm line, the contribution of acoustic waves to the heating of quiet and plage regions in the chromosphere is discussed. The energy released by radiative losses is compared with the energy deposited by acoustic waves. Radiative losses are computed using a grid of six semi-empirical models VAL A--F. The deposited acoustic flux is calculated using power spectra of Doppler oscillations measured in the Ca~II line core. The comparison shows that the spatial correlation of maps of radiative losses and acoustic flux is 70 %. The deposited acoustic flux provides at least 25--30~% of the energy radiated in the quiet chromosphere and 50~% in plage regions. Title: Dynamics of the solar atmosphere above a pore with a light bridge Authors: Sobotka, M.; Švanda, M.; Jurčák, J.; Heinzel, P.; Del Moro, D.; Berrilli, F. Bibcode: 2013A&A...560A..84S Altcode: 2013arXiv1309.7790S Context. Solar pores are small sunspots lacking a penumbra that have a prevailing vertical magnetic-field component. They can include light bridges at places with locally reduced magnetic field. Like sunspots, they exhibit a wide range of oscillatory phenomena.
Aims: A large isolated pore with a light bridge (NOAA 11005) is studied to obtain characteristics of a chromospheric filamentary structure around the pore, to analyse oscillations and waves in and around the pore, and to understand the structure and brightness of the light bridge.
Methods: Spectral imaging observations in the line Ca II 854.2 nm and complementary spectropolarimetry in Fe I lines, obtained with the DST/IBIS spectrometer and HINODE/SOT spectropolarimeter, were used to measure photospheric and chromospheric velocity fields, oscillations, waves, the magnetic field in the photosphere, and acoustic energy flux and radiative losses in the chromosphere.
Results: The chromospheric filamentary structure around the pore has all important characteristics of a superpenumbra: it shows an inverse Evershed effect and running waves, and has a similar morphology and oscillation character. The granular structure of the light bridge in the upper photosphere can be explained by radiative heating. Acoustic waves leaking up from the photosphere along the inclined magnetic field in the light bridge transfer enough energy flux to balance the entire radiative losses of the light-bridge chromosphere.
Conclusions: A penumbra is not a necessary condition for the formation of a superpenumbra. The light bridge is heated by radiation in the photosphere and by acoustic waves in the chromosphere. Title: Tomography of Plasma Flows in the Upper Solar Convection Zone Using Time-Distance Inversion Combining Ridge and Phase-speed Filtering Authors: Švanda, Michal Bibcode: 2013ApJ...775....7S Altcode: 2013arXiv1307.4836S The consistency of time-distance inversions for horizontal components of the plasma flow on supergranular scales in the upper solar convection zone is checked by comparing the results derived using two k-ω filtering procedures—ridge filtering and phase-speed filtering—commonly used in time-distance helioseismology. I show that both approaches result in similar flow estimates when finite-frequency sensitivity kernels are used. I further demonstrate that the performance of the inversion improves (in terms of a simultaneously better averaging kernel and a lower noise level) when the two approaches are combined together in one inversion. Using the combined inversion, I invert for horizontal flows in the upper 10 Mm of the solar convection zone. The flows connected with supergranulation seem to be coherent only for the top ~5 Mm deeper down there is a hint of change of the convection scales toward structures larger than supergranules. Title: Comparison of Solar Surface Flows Inferred from Time-Distance Helioseismology and Coherent Structure Tracking Using HMI/SDO Observations Authors: Švanda, Michal; Roudier, Thierry; Rieutord, Michel; Burston, Raymond; Gizon, Laurent Bibcode: 2013ApJ...771...32S Altcode: 2013arXiv1305.0875S We compare measurements of horizontal flows on the surface of the Sun using helioseismic time-distance inversions and coherent structure tracking of solar granules. Tracking provides two-dimensional horizontal flows on the solar surface, whereas the time-distance inversions estimate the full three-dimensional velocity flows in the shallow near-surface layers. Both techniques use Helioseismic and Magnetic Imager observations as input. We find good correlations between the various measurements resulting from the two techniques. Further, we find a good agreement between these measurements and the time-averaged Doppler line-of-sight velocity, and also perform sanity checks on the vertical flow that resulted from the three-dimensional time-distance inversion. Title: Time-distance inversions for horizontal and vertical flows on supergranular scales applied to MDI and HMI data Authors: Švanda, M.; Schunker, H.; Burston, R. Bibcode: 2013JPhCS.440a2024S Altcode: 2013arXiv1302.0790S We study the possibility of consistent extension of MDI full-disc helioseismic campaigns with the growing data set of HMI observations. To do so, we down-sample and filter the HMI Dopplegrams so that the resulting spatial power spectrum is similar to the spatial power spectrum of MDI full-disc Dopplergrams. The set of co-spatial and co-temporal datacube pairs from both instruments containing no missing and no bad frames were processed using the same codes and inverted independently for all three components of the plasma flow in the near surface layers. The results from the two instruments are highly correlated, however systematically larger (by ~ 20%) flow magnitudes are derived from HMI. We comment that this may be an effect of the different formation depth of the Doppler signal from the two instruments. Title: Atmosphere above a large solar pore Authors: Sobotka, M.; Švanda, M.; Jurčák, J.; Heinzel, P.; Del Moro, D. Bibcode: 2013JPhCS.440a2049S Altcode: 2013arXiv1302.4893S A large solar pore with a granular light bridge was observed on October 15, 2008 with the IBIS spectrometer at the Dunn Solar Telescope and a 69-min long time series of spectral scans in the lines Ca II 854.2 nm and Fe I 617.3 nm was obtained. The intensity and Doppler signals in the Ca II line were separated. This line samples the middle chromosphere in the core and the middle photosphere in the wings. Although no indication of a penumbra is seen in the photosphere, an extended filamentary structure, both in intensity and Doppler signals, is observed in the Ca II line core. An analysis of morphological and dynamical properties of the structure shows a close similarity to a superpenumbra of a sunspot with developed penumbra. A special attention is paid to the light bridge, which is the brightest feature in the pore seen in the Ca II line centre and shows an enhanced power of chromospheric oscillations at 3-5 mHz. Although the acoustic power flux in the light bridge is five times higher than in the "quiet" chromosphere, it cannot explain the observed brightness. Title: Comparison of solar horizontal velocity fields from SDO/HMI and Hinode data Authors: Roudier, Th.; Rieutord, M.; Prat, V.; Malherbe, J. M.; Renon, N.; Frank, Z.; Švanda, M.; Berger, T.; Burston, R.; Gizon, L. Bibcode: 2013A&A...552A.113R Altcode: 2013arXiv1303.4271R Context. The measurement of the Sun's surface motions with a high spatial and temporal resolution is still a challenge.
Aims: We wish to validate horizontal velocity measurements all over the visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic and Magnetic Imager (SDO/HMI) data.
Methods: Horizontal velocity fields are measured by following the proper motions of solar granules using a newly developed version of the coherent structure tracking (CST) code. The comparison of the surface flows measured at high spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI, 0.5 arcsec) allows us to determine corrections to be applied to the horizontal velocity measured from HMI white light data.
Results: We derive horizontal velocity maps with spatial and temporal resolutions of respectively 2.5 Mm and 30 min. From the two components of the horizontal velocity vx and vy measured in the sky plane and the simultaneous line of sight component from SDO/HMI dopplergrams vD, we derive the spherical velocity components (vr, vθ, vϕ). The azimuthal component vϕ gives the solar differential rotation with a high precision (± 0.037 km s-1) from a temporal sequence of only three hours.
Conclusions: By following the proper motions of the solar granules, we can revisit the dynamics of the solar surface at high spatial and temporal resolutions from hours to months and years with the SDO data. Title: An Average Supergranule: Much Larger Vertical Flows Than Expected Authors: Švanda, M. Bibcode: 2013CEAB...37..447S Altcode: 2013arXiv1301.1821S Supergranules are believed to be an evidence for large-scale subsurface convection. The vertical component of the supergranular flow field is very hard to measure, but it is considered to be only a few mps in and below the photosphere. Here I present the results of the analysis using three-dimensional inversion for time-distance helioseismology that indicates existence of large-magnitude vertical upflow in the near sub-surface layers. Possible issues and consequences of this inference are also discussed. Title: A New Look into the Spectral and Light Variations of \varepsilon Aur Authors: Harmanec, P.; Božić, H.; Korčáková, D.; Kotková, L.; Škoda, P.; Šlechta, M.; Švanda, M.; Votruba, V.; Wolf, M.; Zasche, P.; Henden, A.; Ribeiro, J. Bibcode: 2013CEAB...37...99H Altcode: 2013arXiv1303.6401H Investigating long series of spectral and photometric observations, we found that the orbital elements of \varepsilon Aur are subject to much larger uncertainties than usually believed. The Hα emission is found to move basically with the F primary but its exact location should still be investigated. We also find strong additional absorption and large reddening of the object near the third contact during the eclipse. Episodic atmospheric mass transfer from the F primary towards its companion is tentatively suggested. Title: Inversions for Average Supergranular Flows Using Finite-frequency Kernels Authors: Švanda, Michal Bibcode: 2012ApJ...759L..29S Altcode: 2012arXiv1209.6147S I analyze the maps recording the travel-time shifts caused by averaged plasma anomalies under an "average supergranule," constructed by means of statistical averaging over 5582 individual supergranules with large divergence signals detected in two months of Helioseismic and Magnetic Imager Dopplergrams. By utilizing a three-dimensional validated time-distance inversion code, I measure a peak vertical velocity of 117 ± 2 m s-1 at depths around 1.2 Mm in the center of the supergranule and a root-mean-square vertical velocity of 21 m s-1 over the area of the supergranule. A discrepancy between this measurement and the measured surface vertical velocity (a few m s-1) can be explained by the existence of the large-amplitude vertical flow under the surface of supergranules with large divergence signals, recently suggested by Duvall & Hanasoge. Title: Optimisation of solar synoptic observations Authors: Klvaña, Miroslav; Sobotka, Michal; Švanda, Michal Bibcode: 2012SPIE.8448E..0AK Altcode: The development of instrumental and computer technologies is connected with steadily increasing needs for archiving of large data volumes. The current trend to meet this requirement includes the data compression and growth of storage capacities. This approach, however, has technical and practical limits. A further reduction of the archived data volume can be achieved by means of an optimisation of the archiving that consists in data selection without losing the useful information. We describe a method of optimised archiving of solar images, based on the selection of images that contain a new information. The new information content is evaluated by means of the analysis of changes detected in the images. We present characteristics of different kinds of image changes and divide them into fictitious changes with a disturbing effect and real changes that provide a new information. In block diagrams describing the selection and archiving, we demonstrate the influence of clouds, the recording of images during an active event on the Sun, including a period before the event onset, and the archiving of long-term history of solar activity. The described optimisation technique is not suitable for helioseismology, because it does not conserve the uniform time step in the archived sequence and removes the information about solar oscillations. In case of long-term synoptic observations, the optimised archiving can save a large amount of storage capacities. The actual capacity saving will depend on the setting of the change-detection sensitivity and on the capability to exclude the fictitious changes. Title: Some Dynamic Analysis of the Photosphere from Hinode/SOT and SDO/HMI Observations Authors: Roudier, T.; Malherbe, J.; Rieutord, M.; Berger, T.; Frank, Z.; Prat, V.; Renon, N.; Gizon, L.; Svanda, M. Bibcode: 2012ASPC..456...65R Altcode: We first present the important role played by the families of granule (or Tree of Fragmenting granules) in the formation of the photospheric network. Then, we describe the occurence and characteristics of acoustic events (AE), defined as spatially concentrated energy flux, in the quiet Sun. Finally, we present how horizontal velocities obtained from SDO/HMI data are calibrated by using Hinode/SOT observations. Title: Quasi full-disk maps of solar horizontal velocities using SDO/HMI data Authors: Roudier, Th.; Rieutord, M.; Malherbe, J. M.; Renon, N.; Berger, T.; Frank, Z.; Prat, V.; Gizon, L.; Švanda, M. Bibcode: 2012A&A...540A..88R Altcode: 2012arXiv1203.0514R
Aims: For the first time, the motion of granules (solar plasma on the surface on scales larger than 2.5 Mm) has been followed over the entire visible surface of the Sun, using SDO/HMI white-light data.
Methods: Horizontal velocity fields are derived from image correlation tracking using a new version of the coherent structure tracking algorithm. The spatial and temporal resolutions of the horizontal velocity map are 2.5 Mm and 30 min, respectively.
Results: From this reconstruction, using the multi-resolution analysis, one can obtain to the velocity field at different scales with its derivatives such as the horizontal divergence or the vertical component of the vorticity. The intrinsic error on the velocity is ~0.25 km s-1 for a time sequence of 30 min and a mesh size of 2.5 Mm. This is acceptable compared to the granule velocities, which range between 0.3 km s-1 and 1.8 km s-1. A high correlation between velocities computed from Hinode and SDO/HMI has been found (85%). From the data we derive the power spectrum of the supergranulation horizontal velocity field, the solar differential rotation, and the meridional velocity. Title: Multichannel Three-Dimensional SOLA Inversion for Local Helioseismology Authors: Jackiewicz, J.; Birch, A. C.; Gizon, L.; Hanasoge, S. M.; Hohage, T.; Ruffio, J. -B.; Švanda, M. Bibcode: 2012SoPh..276...19J Altcode: 2011arXiv1109.2712J Inversions for local helioseismology are an important and necessary step for obtaining three-dimensional maps of various physical quantities in the solar interior. Frequently, the full inverse problems that one would like to solve prove intractable because of computational constraints. Due to the enormous seismic data sets that already exist and those forthcoming, this is a problem that needs to be addressed. To this end, we present a very efficient linear inversion algorithm for local helioseismology. It is based on a subtractive optimally localized averaging (SOLA) scheme in the Fourier domain, utilizing the horizontal-translation invariance of the sensitivity kernels. In Fourier space the problem decouples into many small problems, one for each horizontal wave vector. This multichannel SOLA method is demonstrated for an example problem in time-distance helioseismology that is small enough to be solved both in real and Fourier space. We find that both approaches are successful in solving the inverse problem. However, the multichannel SOLA algorithm is much faster and can easily be parallelized. Title: Solar synoptic telescope. Characteristics, possibilities, and limits of design Authors: Klvaňa, M.; Sobotka, M.; Švanda, M. Bibcode: 2011CoSka..41...92K Altcode: A rapid evolution of electronics and information technologies makes it possible to use new original designs of synoptic telescopes for solar observations, to increase the demands on their functions, and to fully automate the observation. However, there are hardware and software limits that strongly influence the working capabilities of synoptic telescopes. In this contribution, we analyze relationships between the synoptic telescope's characteristics, the parameters of image digitization, the control, the achievable degree of automation of observations, and the possibilities to implement functions connected with the solar activity monitoring and image archiving. The principles listed above serve as a basis for the design study of the Auxiliary Full-Disc Telescope for the European Solar Telescope (EST), a pan-European project of a large 4-meter solar telescope. Title: Validated helioseismic inversions for 3D vector flows Authors: Švanda, M.; Gizon, L.; Hanasoge, S. M.; Ustyugov, S. D. Bibcode: 2011A&A...530A.148S Altcode: 2011arXiv1104.4083S Context. According to time-distance helioseismology, information about internal fluid motions is encoded in the travel times of solar waves. The inverse problem consists of inferring three-dimensional vector flows from a set of travel-time measurements. While only few tests of the inversions have been done, it is known that the retrieval of the small-amplitude vertical flow velocities is problematic. A thorough study of biases and noise has not been carried out in realistic conditions.
Aims: Here we investigate the potential of time-distance helioseismology to infer three-dimensional convective velocities in the near-surface layers of the Sun. We developed a new subtractive optimally localised averaging (SOLA) code suitable for pipeline pseudo-automatic processing. Compared to its predecessor, the code was improved by accounting for additional constraints in order to get the right answer within a given noise level. The main aim of this study is to validate results obtained by our inversion code.
Methods: We simulate travel-time maps using a snapshot from a numerical simulation of solar convective flows, realistic Born travel-time sensitivity kernels, and a realistic model of travel-time noise. These synthetic travel times are inverted for flows and the results compared with the known input flow field. Additional constraints are implemented in the inversion: cross-talk minimization between flow components and spatial localization of inversion coefficients.
Results: Using modes f, p1 through p4, we show that horizontal convective flow velocities can be inferred without bias, at a signal-to-noise ratio greater than one in the top 3.5 Mm, provided that observations span at least four days. The vertical component of velocity (vz), if it were to be weak, is more difficult to infer and is seriously affected by cross-talk from horizontal velocity components. We emphasise that this cross-talk must be explicitly minimised in order to retrieve vz in the top 1 Mm. We also show that statistical averaging over many different areas of the Sun allows for reliably measuring of average properties of all three flow components in the top 5.5 Mm of the convection zone.

Figures 16-28 are available in electronic form at http://www.aanda.org Title: Space-time segmentation method for study of the vertical structure and evolution of solar supergranulation from data provided by local helioseismology Authors: Žlebčík, R.; Švanda, M.; Klvaňa, M. Bibcode: 2011NewA...16....1Z Altcode: 2010arXiv1006.0617Z Solar supergranulation remains a mystery in spite of decades of intensive studies. Most of the papers about supergranulation deal with its surface properties. Local helioseismology provides an opportunity to look below the surface and see the vertical structure of this convective feature. We present a concept of a (3 + 1)-D segmentation algorithm capable of recognising individual supergranules in a sequence of helioseismic 3-D flow maps. As an example, we applied this method to the state-of-the-art data and derived descriptive statistical properties of segmented supergranules - typical size of 20-30 Mm, characteristic lifetime of 18.7 h, and estimated depth of 15-20 Mm. We present preliminary results obtained on the topic of the three-dimensional structure and evolution of supergranulation. The method has great potential in analysing the better data expected from the helioseismic inversions, which are being developed. Title: Dynamics of Active Regions Revealed by Tracking of Doppler Features Authors: Švanda, M.; Sobotka, M.; Klvaňa, M.; Bumba, V. Bibcode: 2010ASSP...19..410S Altcode: 2010mcia.conf..410S We investigate the large-scale horizontal dynamics of active regions in the 23rd solar cycle. The large-scale horizontal velocity fields were measured applying the local correlation tracking (LCT) algorithm to the processed high-cadence full-resolution full-disc MDI Dopplergrams. We performed the selection of NOAA active regions in the available dataset and followed their individual evolution in time. The statistical study of this sample gives us a unique opportunity to study the dynamics of active regions at various stages of their evolution. In few cases, we found behavior that is consistent with the dynamical disconnection of sunspots from the magnetic roots. Title: Transport of Supergranules and their Vertical Coherence Authors: Švanda, M.; Kosovichev, A. G.; Klvaňa, M.; Sobotka, M.; Duvall, T. L., Jr. Bibcode: 2009ASPC..416..547S Altcode: In recent papers, we have introduced a method for measuring the photospheric flow field that is based on the tracking of supergranular structures. Here, in combination with helioseismic data, we are able to estimate the depth in the solar convection envelope to which the detected large-scale flow field is coherent. We show that the upper 10 Mm in the convection zone depicts similar features in horizontal velocity. Our interpretation of this observation is that the supergranulation is a coherent structure 10 Mm deep and is subject to large-scale transport by the underlying velocity field. Title: Large-scale horizontal flows in the solar photosphere. V. Possible evidence for the disconnection of bipolar sunspot groups from their magnetic roots Authors: Švanda, M.; Klvaňa, M.; Sobotka, M. Bibcode: 2009A&A...506..875S Altcode: 2009arXiv0908.3183S In a recent paper (Švanda et al. 2008, A&A, 477, 285) we pointed out that, based on the tracking of Doppler features in the full-disc MDI Dopplergrams, the active regions display two dynamically different regimes. We speculated that this could be a manifestation of the sudden change in the active regions dynamics, caused by the dynamic disconnection of sunspots from their magnetic roots as proposed by Schüssler & Rempel (2005, A&A, 441, 337). Here we investigate the dynamic behaviour of the active regions recorded in the high-cadence MDI data over the last solar cycle in order to confirm the predictions in the Schüssler's & Rempel's paper. We find that, after drastic reduction of the sample, which is done to avoid disturbing effects, a large fraction of active regions displays a sudden decrease in the rotation speed, which is compatible with the mechanism of the dynamic disconnection of sunspots from their parental magnetic structures. Title: Large-scale horizontal flows in the solar photosphere IV. On the vertical structure of large-scale horizontal flows Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Kosovichev, A. G.; Duvall, T. L. Bibcode: 2009NewA...14..429S Altcode: 2008arXiv0812.1971S In the recent papers, we introduced a method utilised to measure the flow field. The method is based on the tracking of supergranular structures. We did not precisely know, whether its results represent the flow field in the photosphere or in some subphotospheric layers. In this paper, in combination with helioseismic data, we are able to estimate the depths in the solar convection envelope, where the detected large-scale flow field is well represented by the surface measurements. We got a clear answer to question what kind of structures we track in full-disc Dopplergrams. It seems that in the quiet Sun regions the supergranular structures are tracked, while in the regions with the magnetic field the structures of the magnetic field are dominant. This observation seems obvious, because the nature of Doppler structures is different in the magnetic regions and in the quiet Sun. We show that the large-scale flow detected by our method represents the motion of plasma in layers down to ∼10 Mm. The supergranules may therefore be treated as the objects carried by the underlying large-scale velocity field. Title: Photospheric flows around a quiescent filament at Large and small scale and their ffects on filament destabilization Authors: Roudier, Th.; Malherbe, J. M.; Švanda, M.; Molodij, G.; Keil, S.; Sütterlin, P.; Schmieder, B.; Bommier, V.; Aulanier, G.; Meunier, N.; Rieutord, M.; Rondi, S. Bibcode: 2008sf2a.conf..569R Altcode: We study the influence of large and small scales photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004 as part of an international observing campaign (JOP 178). Large-scale horizontal flows are invetigated from a series of MDI/SOHO full-disc Dopplergrams and magnetograms from THEMIS. Small-scale horizontal flows were derived using local correlation tracking on TRACE satellite, Dutch Open Telescope (DOT) and The Dunn Solar telescope (DST) data. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase of the shear below the point where the eruption starts and a decrease in shear after the eruption. We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament. Title: Tracking of Supergranules - Does It Make Any Sense? Authors: Svanda, M.; Klvaòa, M.; Sobotka, M. Bibcode: 2008ESPM...12.2.10S Altcode: 2008arXiv0809.4757S The motions of the plasma and structures in and below the solar photosphere is not well understood. The results obtained using various methods cannot be in general considered as consistent, especially in details. In this contribution we show a summary of the results obtained by the method we have developed recently.

To study the photospheric dynamics we apply the local correlation tracking algorithm to the series of full-disc Dopplergrams obtained by Michelson Doppler Imager (MDI) on-board the SoHO observatory. The dominant structures recorded in the Dopplergrams are supergranules. Under the assumtion that the supergranules are carried by the flow field of the larger scale, we study properties of this underlying velocity field. The methodology consists of an extensive data processing of primary data in order to suppress disturbing effects such as p-modes of solar oscillations or instrumental issues. Aditional coordinate transformations are also needed to make the data suitable for tracking.

We perform comparative tests with synthetic data with known properties and with results of time-distance helioseismology with a great success. Correlation coeficients of the comparison of mean components of the flow field are larger than 0.8, for the comparison of details in the vector velocity field the correlation coeficient is larger than 0.6.

The results of the method applied to the real data agree well with well-known features detected in the photospheric velocity fields and reported by many other authors. With the proposed method we detect differential rotation, meridional circulation, torsional oscillations, and other features. A few case studies are shown to demonstrate the performance of the method.

As a conclusion let's answer the question in the title. We believe that tracking of supergranules makes a perfect sense when studying the large-scale flows in the solar photosphere. The method we demonstrate is suitable to detect large-scale velocity field with effective resolution of 60" and random error of 15 m/s. We believe that our method may provide a powerful tool for studies related to the dynamic behaviour of plasmas in the solar photosphere. Title: The Conception of the Full-disc Telescope for EST Instrument Authors: Klvana, M.; Sobotka, M.; Svanda, M. Bibcode: 2008ESPM...12.2.73K Altcode: In year 2008 European Association for Solar Telescopes (EAST) started to carry out the project of the European Solar Telescope (EST) instrument. The main telescope of EST is expected to have a diameter of 4 m and therefore it will become one of the biggest solar telescopes in the world. The main telescope will be connected to the smaller full-disc solar telescope, the development of which is carried out by our group.

In this contribution we present optical and mechanical demands of the telescope, its optical design and the description of the basic operation regimes. We analyse the possibilities of inclusion of this full-disc telescope in the general EST controlling system. We show that the small telescope can be used for alignment of the main telescope and for the determination of the changing instrumental constants of the instrument during a long-term run. Title: Effects of Solar Active Regions on Meridional Flows Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei Bibcode: 2008ApJ...680L.161S Altcode: 2008arXiv0805.1789S The aim of this Letter is to extend our previous study of the solar-cycle variations of meridional flows and to investigate their latitudinal and longitudinal structure in the subphotospheric layer, especially their variations in magnetic regions. Helioseismology observations indicate that mass flows around active regions are dominated by inflows into those regions. On average, those local flows are more important around the leading magnetic polarities of active regions than around the following polarities and depend on the evolutionary stage of particular active regions. We present a statistical study based on MDI/SOHO observations of 1996-2002 and show that this effect explains a significant part of the cyclic change of meridional flows in near-equatorial regions, but not at higher latitudes. A different mechanism driving solar-cycle variations of the meridional flow probably operates. Title: Large-scale horizontal flows in the solar photosphere. III. Effects on filament destabilization Authors: Roudier, T.; Švanda, M.; Meunier, N.; Keil, S.; Rieutord, M.; Malherbe, J. M.; Rondi, S.; Molodij, G.; Bommier, V.; Schmieder, B. Bibcode: 2008A&A...480..255R Altcode: 2007arXiv0711.3112R Aims:We study the influence of large-scale photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004, as part of an international observing campaign (JOP 178).
Methods: Large-scale horizontal flows were investigated from a series of MDI full-disc Dopplergrams and magnetograms. From the Dopplergrams, we tracked supergranular flow patterns using the local correlation tracking (LCT) technique. We used both LCT and manual tracking of isolated magnetic elements to obtain horizontal velocities from magnetograms.
Results: We find that the measured flow fields obtained by the different methods are well-correlated on large scales. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase in the shear below the point where the eruption starts and a decrease in shear after the eruption. We find a pattern in the large-scale horizontal flows at the solar surface that interact with differential rotation.
Conclusions: We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament. Title: Large-scale horizontal flows in the solar photosphere. II. Long-term behaviour and magnetic activity response Authors: Švanda, M.; Klvaňa, M.; Sobotka, M.; Bumba, V. Bibcode: 2008A&A...477..285S Altcode: 2007arXiv0710.2693S We have developed a method to map large-scale horizontal velocity fields in the solar photosphere. The method was developed, tuned, and calibrated using synthetic data. Now, we apply the method to the series of Michelson Doppler Imager (MDI) Dopplergrams covering almost one solar cycle to generate the information about the long-term behaviour of surface flows. Our method clearly reproduces the widely accepted properties of mean flow field components, such as torsional oscillations and a pattern of meridional circulation. We also performed a period analysis, however due to the data series length and large gaps we did not detect any significant periods. The relation between the magnetic activity and the mean zonal motion is studied. We found evidence that the emergence of compact magnetic regions locally accelerates the rotation of the supergranular pattern in their vicinity and that the presence of magnetic fields generally decelerates the rotation in the equatorial region. Our results show that active regions in the equatorial region emerge, exhibiting a constant velocity (faster by 60 ± 9 m s-1 than the Carrington rate), suggesting that they emerge from the base of the surface radial shear at 0.95 R_⊙, disconnect from their magnetic roots, and slow down during their evolution. Title: Surface velocity network with anti-solar differential rotation on the active K-giant σ Geminorum Authors: Kovári, Zs.; Bartus, J.; Švanda, M.; Vida, K.; Strassmeier, K. G.; Oláh, K.; Forgács-Dajka, E. Bibcode: 2007AN....328.1081K Altcode: 2007arXiv0711.0771K We demonstrate the power of the local correlation tracking technique on stellar data for the first time. We recover the spot migration pattern of the long-period RS CVn-type binary σ Gem from a set of six Doppler images from 3.6 consecutive rotation cycles. The resulting surface flow map suggests a weak anti-solar differential rotation with {α≈-0.0022±0.0016}, and a coherent poleward spot migration with an average velocity of 220±10 m s-1. This result agrees with our recent findings from another study and could also be confirmed theoretically. Title: Velocity Fields in the Solar Photosphere Authors: Svanda, Michal Bibcode: 2007arXiv0712.1958S Altcode: Large-scale velocity fields in the solar photosphere remain a mystery in spite of many years of intensive studies. In this thesis, the new method of the measurements of the solar photospheric flow fields is proposed. It is based on local correlation tracking algorithm applied to full-disc dopplergrams obtained by Michelson Doppler Images (MDI) on-board the Solar and Heliospheric Observatory (SoHO). The method is tuned and tested on synthetic data, it is shown that the method is capable of measuring of horizontal velocity fields with an accuracy of 15 \mps. It is also shown that the method provides the measurements comparable with time-distance local helioseismology. The method is applied to real data sets. It reproduces well known properties of solar photospheric velocity fields. Moreover, the case studies show an evidence about the influence of the changes in the flow field topology on the stability of the eruptive filament and support the theory of the dynamical disconnection of bipolar sunspots from their magnetic roots. The method has a great perspective in the future use. The meridional flux transportation speed is also studied and it is shown that the direct measurement may differ from time-distance local helioseimology in the areas occupied by the strong magnetic field. This result has an impact to the flux transport dynamo models, which use the meridional speed as the essential observational input parameter. Title: Anti-solar differential rotation and surface flow pattern on UZ Libræ Authors: Vida, K.; Kovári, Zs.; Švanda, M.; Oláh, K.; Strassmeier, K. G.; Bartus, J. Bibcode: 2007AN....328.1078V Altcode: 2007arXiv0711.1187V We re-investigate UZ Libræ spectra obtained at KPNO in 1998 and 2000. From the 1998 data we compose 11 consecutive Doppler images using the Ca I-6439, Fe I-6393 and Fe I-6411 lines. Applying the method of average cross-correlation of contiguous Doppler images we find anti-solar differential rotation with a surface shear of α≈-0.03. The pilot application of the local correlation tracking technique for the same data qualitatively confirms this result and indicates complex flow pattern on the stellar surface. From the cross-correlation of the two available Doppler images in 2000 we also get anti-solar differential rotation but with a much weaker shear of α≈-0.004. Title: Speed of Meridional Flows and Magnetic Flux Transport on the Sun Authors: Švanda, Michal; Kosovichev, Alexander G.; Zhao, Junwei Bibcode: 2007ApJ...670L..69S Altcode: 2007arXiv0710.0590S We use the magnetic butterfly diagram to determine the speed of the magnetic flux transport on the solar surface toward the poles. The manifestation of the flux transport is clearly visible as elongated structures extended from the sunspot belt to the polar regions. The slopes of these structures are measured and interpreted as meridional magnetic flux transport speed. Comparison with the time-distance helioseismology measurements of the mean speed of the meridional flows at a depth of 3.5-12 Mm shows a generally good agreement, but the speeds of the flux transport and the meridional flow are significantly different in areas occupied by the magnetic field. The local circulation flows around active regions, especially the strong equatorward flows on the equatorial side of active regions, affect the mean velocity profile derived by helioseismology but do not influence the magnetic flux transport. The results show that the mean longitudinally averaged meridional flow measurements by helioseismology may not be used directly in solar dynamo models for describing the magnetic flux transport, and that it is necessary to take into account the longitudinal structure of these flows. Title: Anti-solar differential rotation on the active K-giant σ Geminorum Authors: Kovári, Zs.; Bartus, J.; Strassmeier, K. G.; Vida, K.; Švanda, M.; Oláh, K. Bibcode: 2007A&A...474..165K Altcode: The active K1 giant σ Gem and its differential surface rotation is revisited. We refine our previous inconclusive result by recovering the spot migration pattern of this long-period RS CVn-type binary through application of the technique of “average cross-correlation of contiguous Doppler images” to a set of six Doppler images from 3.6 consecutive rotation cycles. We find an anti-solar differential rotation law with a shear of α≈ -0.022±0.006. We also find evidence of a poleward migration trend of spots with an average velocity of ≈300 m s-1. Title: Comparison of Large-Scale Flows on the Sun Measured by Time-Distance Helioseismology and Local Correlation Tracking Authors: Švanda, Michal; Zhao, Junwei; Kosovichev, Alexander G. Bibcode: 2007SoPh..241...27S Altcode: 2007astro.ph..1717S We present a direct comparison between two different techniques: time-distance helioseismology and a local correlation tracking method for measuring mass flows in the solar photosphere and in a near-surface layer. We applied both methods to the same dataset (MDI high-cadence Dopplergrams covering almost the entire Carrington rotation 1974) and compared the results. We found that, after necessary corrections, the vector flow fields obtained by these techniques are very similar. The median difference between directions of corresponding vectors is 24°, and the correlation coefficients of the results for mean zonal and meridional flows are 0.98 and 0.88, respectively. The largest discrepancies are found in areas of small velocities where the inaccuracies of the computed vectors play a significant role. The good agreement of these two methods increases confidence in the reliability of large-scale synoptic maps obtained by them. Title: Large-scale horizontal flows in the solar photosphere. I. Method and tests on synthetic data Authors: Švanda, M.; Klvaňa, M.; Sobotka, M. Bibcode: 2006A&A...458..301S Altcode: 2006astro.ph..8443S We propose a useful method for mapping large-scale velocity fields in the solar photosphere. It is based on the local

correlation tracking algorithm when tracing supergranules in full-disc dopplergrams. The method was developed using synthetic data.

The data are transformed during the data processing into a suitable coordinate system, the noise is removed, and finally the velocity field is

calculated. Resulting velocities are compared with the model velocities and the calibration is done. From our results it becomes clear that

this method could be applied to full-disc dopplergrams acquired by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SoHO). Title: Doppler Velocity Fields in Magnetic Structures and their Surroundings Authors: Klvaňa, M.; Bumba, V.; Švanda, M. Bibcode: 2006CEAB...30...21K Altcode: We demonstrate the effect of a velocity shift of photospheric velocity fields in regions filled by magnetic field compared to the velocity fields of non-magnetic regions surrounding them. We analyse this effect on five different types of measurements in the active region NOAA 8086 obtained by SOLMAG (Ondřejov magnetograph) in four spectral lines on September 18, 1997. The mean value of the Doppler velocity field in regions filled by magnetic field shows a red-shift of 40--190 m s-1 with respect to the neighbouring non-magnetic regions. Title: Mapping of Large-Scale Photospheric Velocity Fields Authors: Švanda, M.; Klvaňa, M.; Sobotka, M. Bibcode: 2005ESASP.600E..71S Altcode: 2005ESPM...11...71S; 2005dysu.confE..71S No abstract at ADS Title: Temporal Changes of the Photospheric Velocity Fields Authors: Klvaňa, M.; Švanda, M.; Bumba, V. Bibcode: 2005HvaOB..29...89K Altcode: We analyse the influence of the planets Mercury, Venus, Earth and Jupiter on the Doppler velocity field in the solar photosphere, using the theory of tidal forces. We compare the measured Doppler velocity fields estimated in a zonal area along the solar equator with the results of the dynamical calculations. From this comparison it follows that we do not succeed to demonstrate the presence of a velocity field, caused by the tidal forces, in the measured data. If tidal waves in the solar photosphere do exist, they are lost in the noise and their horizontal velocity field probably will be under the limit of ±20 m s^{-1}. Title: Motions of Supergranular Structures on the Solar Surface Authors: Švanda, M.; Klnaňa, M.; Sobotka, M. Bibcode: 2005HvaOB..29...39S Altcode: We present a method used to study motions of supergranular structures in the solar photosphere. It is based on the local correlation tracking method applied to full-disk dopplergrams measured by SoHO/MDI. In primary dopplergrams, there is a plenty of noise originating mostly from oscillations and morphological changes of the shape of supergranules. We describe a method used to suppress such noise. To demonstrate the suggested technique and properties of computed velocity fields, we processed a few-day period during the time of minimum of solar activity. The obtained vector velocity fields are drawn using streamlines in the Carrington's coordinate system. We discuss the reproductivity of the obtained results and the connection of the velocity field we found to the large-scale velocity field in the upper part of the convection zone. Title: Do tidal waves exist in the solar photosphere? Authors: Klvaňa, M.; Švanda, M.; Krivtsov, A.; Bumba, V. Bibcode: 2004HvaOB..28..157K Altcode: The temporal coincidences between solar activity regularities and the motion of some planets show that there could exist a mutual physical relationship between both processes. One of the possible causal agents could be the action of the gravitational field. Although the altitude of planetary tidal waves on the Sun, following many authors, is of the order of only one millimetre, in the case of resonance it could be enlarged appreciably. Therefore, we decided to verify the possibility of detection of such waves in the solar atmosphere using the earlier developed dynamical theory of tidal waves. In this paper we present and discuss the preliminary results, obtained from the comparison of the velocity fields measured with the MDI instrument of the SOHO satellite and of the velocity fields of a tidal wave calculated on the basis of the dynamical theory. Title: Dynamics of motions in the quiet photosphere Authors: Svanda, M.; Klvana, M.; Sobotka, M.; Bumba, V. Bibcode: 2003ESASP.535..149S Altcode: 2003iscs.symp..149S We determine the vector velocity fields describing the plasma motions in the quiet solar photosphere, using the motions of the supergranular structures, obtained by the analysis of series of Doppler measurements of velocity fields of the whole solar disk. It turned out that the studied vector velocity fields can be submerged under the noise level, originating due to the strong variability of the supergranular structures during their life time. We describe the method we used for the suppressing of such noise, and we bring the criterions used by the election of free parameters. We demonstrate examples of obtained vector velocity fields and of the resulting motions of matter on the visible photospheric surface. We discuss different factors influencing the reproductivness of obtained results. We construct a graph of the dependence of the differential rotation on the heliographic latitude from the mean vector velocity field, we got for the solar disk without expressive magnetic fields. Title: Comparison of Coronal Structures 11.8.1999 on the Long Observation Base Authors: Belík, M.; Markova, E.; Brandejsova, E.; Kucera, J.; Kares, J.; Svanda, M. Bibcode: 2000ESASP.463..587B Altcode: 2000sctc.proc..587B No abstract at ADS