Michal Švanda

Comparison of Large-Scale Flows on the Sun Measured by Time-Distance Helioseismology and Local Correlation Tracking

Abstract 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.

SPEED OF MERIDIONAL FLOWS AND MAGNETIC FLUX TRANSPORT ON THE SUN

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.

Large-scale horizontal flows in the solar photosphere I. Method and tests on synthetic data

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).

Dynamics of the solar atmosphere above a pore with a light bridge

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.

Tomography of Plasma Flows in the Upper Solar Convection Zone Using Time-Distance Inversion Combining Ridge and Phase-speed Filtering

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-? 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.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-{omega} 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 {approx}5 Mm; deeper down there is a hint of change of the convection scales toward structures larger than supergranules.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. It is shown that both approaches result in similar flow estimates when finitefrequency sensitivity kernels are used. It is further demonstrated that the performance of the inversion improves (in terms of simultaneously better averaging kernel and 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 in the upper ∼ 5 Mm depth, deeper down there is a hint on change of convection scales towards structures larger than supergranules. Subject headings: Sun: helioseismology – Sun: interior

INVERSIONS FOR AVERAGE SUPERGRANULAR FLOWS USING FINITE-FREQUENCY KERNELS

I analyse 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 HMI Dopplergrams. By utilising a threedimensional validated time–distance inversion code, I measure the peak vertical velocity of 117±2 m s−1 in depths around 1.2 Mm in the centre of the supergranule and 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 (2012). Subject headings: Sun: helioseismology — Sun: interior 1. LARGE-MAGNITUDE SUBSURFACE SUPERGRANULAR FLOWS? The nature of supergranules – convection-like structures observed in the solar photosphere – is largely still in debates (see a review by Rieutord & Rincon 2010). Only the surface properties of supergranules are well established. The plasma flows within the supergranules are predominantly horizontal with root-mean-square velocity ∼300 m s−1. The vertical component of the supergranular flow is difficult to measure, its amplitude is usually within measurement error bars. Statistically it has been determined that the root-mean-square vertical velocity ranges from 4 m s−1 (Duvall & Birch 2010) to 29 m s−1 (Hathaway et al. 2002). The deep structure of supergranules is practically unknown. Attempts were made using local helioseismic methods with controversial results. Some studies (e.g. Duvall 1998; Zhao & Kosovichev 2003, to name a few) revealed supergranules as convection-like cells extending to depths of 8–25 Mm with a deep “return flow”. Woodard (2007) and Jackiewicz et al. (2008) did not detect the flow reversal, but pointed out that any inversion for the flow snapshot deeper than 4–6 Mm is dominated by the random noise and thus does not reveal any information about the deep supergranular flow. Hathaway (2012) recently showed that supergranules may extend to depths equal to their widths indicating that analysis of supergranules with differing sizes may lead to a different depth structure. Numerical simulations of near-surface Sun-like convection (e.g. Ustyugov 2008) indicated that the amplitude of the flows on supergranular scales decreased with increasing depth, although the flows were highly structured on smaller scales. Recently, a very surprising result came to light from helioseismology – Duvall & Hanasoge (2012) claimed that the careful analysis of travel-time maps provided evidence for a large-amplitude flow under the surface of supergranules. No inversion was performed in this work. The michal@astronomie.cz authors detected a systematic offset in the travel-time shifts measured for large separations between the measurement points using a special spatio-temporal filtering of the data. This non-standard procedure was selected to avoid the cross-talk between the horizontal and vertical flow in supergranules, which was known to be hard to avoid in inverse methods. They concluded that the offset was due to the large-amplitude vertical flow below the surface. Using a simple Gaussian model, constrained by surface measurements, they predicted a peak in the vertical flow of 240 m s−1 at a depth of 2.3 Mm. In this study, I use a recently implemented, improved time–distance inversion code (which minimises the crosstalk between the flow components, the main issue leading Duvall & Hanasoge to use non-standard time–distance methods) to verify their hypothesis.

Comparison of solar surface flows inferred from time-distance helioseismology and coherent structure tracking using HMI/SDO observations

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.

Large-scale horizontal flows in the solar photosphere III. Effects on filament destabilization

LERMA, Observatoire de Paris, Section de Meudon, 92195 Meudon, FranceReceived February 2, 2008/ SubmittedABSTRACTAims. We study the influence of large-scale photospheric motions on the destabilization of an eruptive filament, observed on Oc tober6, 7, and 8, 2004, as part of an international observing campaign (JOP 178).Methods.Large-scale horizontal flows were invetigated from a series of MDI full-disc Dopplergrams and magnetograms. From theDopplergrams, we tracked supergranular flow patterns using the local correlation tracking (LCT) technique. We used both LCT andmanual tracking of isolated magnetic elements to obtain horizontal velocities from magnetograms.Results. We find that the measured flow fields obtained by the di fferent methods are well-correlated on large scales. The topology ofthe flow field changed significantly during the filament erupti ve phase, suggesting a possible coupling between the surface flow fieldand the coronal magnetic field. We measured an increase in the shear below the point where the eruption starts and a decrease in shearafter the eruption. We find a pattern in the large-scale horizontal flows at the solar surface that interact with di fferential rotation.Conclusions. We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptivefilament.Key words. The Sun: Atmosphere – The Sun: Filaments – The Sun: Magnetic fi elds

Comparison of solar horizontal velocity fields from SDO/HMI and Hinode data

The measurement of the Suns surface motions with a high spatial and temporal resolution is still a challenge. 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. 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. 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 v_D, we derive the spherical velocity components (Vr, Vtheta, Vphi). The azimuthal component Vphi gives the solar differential rotation with a high precision (+-0.037km/s) from a temporal sequence of only three hours. 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.

EFFECTS OF SOLAR ACTIVE REGIONS ON MERIDIONAL FLOWS

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.