Michal Sobotka

Temporal Evolution of Fine Structures in and around Solar Pores

Time series of high-resolution white-light images of six solar pores, observed in 1993 and 1995 at the Swedish Vacuum Solar Telescope (La Palma), are analyzed. The pores constitute an almost ideal laboratory in which to study the interaction of a vertical magnetic field with surrounding convective motions, without the perturbation of the inclined magnetic field in the penumbra. Umbral dots observed in a large (D=89) pore are similar to those in mature umbrae, but they live longer, are brighter, and have a higher filling factor. Granular motions in the vicinity of pores are driven by mesogranular flows. Motions toward the pore dominate in the 2 zone around the pore boundary, while at larger distances the granules move away from the pore. Pushed by these motions, small granules and granular fragments located close to the pore border sometimes penetrate into the pore, where they move inward as bright short-lived features very similar to umbral dots. The capture of bright features by the pore is probably a microscale manifestation of the turbulent erosion, which results in the decay of the pore. Formation of a transitory penumbra-like structure at the border of the large pore was observed simultaneously with the appearance of expanding elongated granules, separated by dark filaments, in an adjacent granular field. These effects can be interpreted as a consequence of emerging bipolar magnetic loops caused by a temporary protrusion of opposite magnetic polarity.

A High-Resolution Study of Inhomogeneities in Sunspot Umbrae

A detailed study of the brightness, size, spatial distribution, and filling factor of the different umbral inhomogeneities is presented. Many sunspots show multiple umbrae (separated by strong light bridges, SLBs), which behave like independent units. These are termed umbral cores (UCs). From the phenomenological point of view, UCs show two components: (1) a dark area, formed by a diffuse background (DB), with local intensity minima called dark nuclei (DNs), and (2) bright features including umbral dots (UDs) and faint light bridges (FLBs)

Narrowband dm-spikes in the 2 GHz frequency range and MHD cascading waves in reconnection outflows

Two examples of clouds of narrowband dm-spikes, observed by the Ondřejov radiospectrograph in the 1–2 GHz frequency range, are analyzed. After transformation of the frequency scales to distances in the solar atmosphere, the power spectra analysis of size scales reveals a spectral index of −5/3, resembling that of Kolmogorov spectra of turbulent cascades. The narrowband dm-spikes are interpreted as radio emission from electrons accelerated in MHD cascading waves, probably generated in plasma outflows from magnetic field reconnection.

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).

Infrared Photometry of Solar Photospheric Structures. II. Center-to-Limb Variation of Active Regions

Simultaneous broadband single images and time series of images have been obtained at λ0.8 and λ1.55 μm, which are at the extremes of the continuum absorption coefficient of the solar atmosphere. Active regions have been observed for different heliocentric angles. Weighted difference images between the two wavelength bands have been used to identify faculae. Center-to-limb variations of facular relative intensity distribution, facular size distribution, and facular size-intensity relations have been obtained. At the spatial resolution of the observations, faculae show no contrast at the disk center at λ0.8 μm, while dark faculae are observed at λ1.55 μm. The transition from dark to bright faculae occurs between μ = 0.6 and 0.5 in λ1.55 μm images. The maximum of the mean facular relative intensity is found at μ = 0.3 for both wavelengths. However, the peak of the facular relative intensity is greater and appears closer to the limb the larger the faculae are. Brightness temperature maps have been computed for the best pairs of images, and temperature difference images have been derived. The temperature difference Tb(1.55 μm) - Tb(0.8 μm) in pores is larger than that in the quiet photosphere at the disk center, but smaller near the limb. Faculae show smaller temperature differences than the quiet photosphere at the disk center, but the temperature differences near the limb are almost equal. Pores are surrounded by ringlike structures of low temperature difference at the disk center. Near the limb these ringlike structures appear bright in the brightness temperature maps and show a temperature difference similar to that of the quiet photosphere.

Power-law spectra of 1–2 GHz narrowband dm-spikes

Twelve examples of clouds of narrowband dm-spikes, observed by the Ondřejov radiospectrograph in the 1–2xa0GHz frequency range, are analyzed. After transforming of the frequency scales to heights in the solar atmosphere, the indices of the power-law power spectra are determined. The derived power-law indices are scattered in a broad range of values (−0.80–−2.85). In some cases they considerably deviate from the previously found value of −5/3. A change of the power-law index above logu2009k≈2.5 was also found in some cases. In the two longest events the time evolution of their power spectra as well as their indices were studied. While in most parts of the radio spectra the spectral index remains constant, in one part its absolute value increases with the spike intensity increase. Finally, the results, especially the broad range of power-law indices, are briefly discussed.

Infrared Photometry of Solar Photospheric Structures. I. Active Regions at the Center of the Disk

Simultaneous time series of broadband images of two active regions close to the disk center were acquired at the Swedish Vacuum Solar Telescope, La Palma, in the infrared bands at 1.55 and 0.80 μm, corresponding to the minimum and maximum continuum opacities, respectively. Dark faculae are detected in images obtained as weighted intensity differences between both wavelength bands. Maps of brightness temperatures Tb (1.55 μm) and Tb (0.80 μm) were computed for the best pairs of images. In the scatter plots Tb (1.55) versus Tb (0.80), the elements of quiet regions can be clearly distinguished from those of faculae and pores, while the transition between faculae and pores is smooth. The temperature difference Tb(1.55) - Tb(0.80) in faculae is lower than that in the quiet photosphere but increases with decreasing Tb and is higher inside pores. Most of the pores are surrounded by ringlike regions of low temperature difference. The minimum intensity of pores at both wavelengths decreases with increasing diameter. Maps of horizontal motions of dark faculae and pores were derived from time series of intensity-difference images, using the local correlation tracking technique. Velocities corresponding to large-scale separation of polarities, an emergence of magnetic flux, twist and contraction related to a pore formation, shear motions, and a twist in dark faculae were measured.

Large-scale horizontal flows in the solar photosphere V: Possible evidence for the disconnection of bipolar sunspot groups from their magnetic roots

In a recent paper (Svanda 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 Schuessler & 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 Schuesslers & Rempels 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.

MOAT FLOW SYSTEM AROUND SUNSPOTS IN SHALLOW SUBSURFACE LAYERS

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.

The Temperature – Magnetic Field Relation in Observed and Simulated Sunspots

Observations of the relation between continuum intensity and magnetic field strength in sunspots have been made for nearly five decades. This work presents full-Stokes measurements of the full-split (g=3