M. Sobotka

Time Series of Solar Granulation Images. I. Differences between Small and Large Granules in Quiet Regions

A 90 minute time series of high spatial resolution white-light images of solar granulation, obtained at the Swedish Vacuum Solar Tower (Observatorio del Roque de los Muchachos, La Palma), was analyzed to study how the physical properties of the granules changed with size. The observational material was corrected for global motions and for the instrumental profile, and a subsonic filter was applied. A definition of granular border was adopted using the inflection points of the intensity of the images, and the granular cells were defined as areas including, in addition to the granules, one-half of their surrounding intergranular lanes. Using time series to investigate the average behavior of solar granulation has three strong advantages: the first is the possibility of removing the acoustic waves; second, the possibility of estimating the effect of the variability of seeing on our results; and, third, the opportunity to attain high statistical significance in the analysis as a result of the large number of extracted granules (61,138). It is shown that the granules of the sample can be classified according to their mean and maximum intensities and their fractal dimension into two regimes, with diameters smaller than and larger than 14, respectively. A broad transition region in which both regimes coexist was found. The resolved internal brightness structure of both the granules and the intergranular lanes shows a linear increase of the number of substructures with the granular and intergranular areas. The diameters of these substructures range between our effective resolution limit (~03) and ~15, with preferential sizes at 065 and 055, respectively. Moreover, it seems that large and small granules are unevenly distributed with respect to the large-scale vertical flows. Thus smaller granules are more concentrated along downdrafts whereas larger ones preferentially occupy the updrafts. Finally, a physical scenario compatible with the existence of these two granular populations is discussed.

The magnetic canopy above light bridges

An analysis of high-resolution Stokes observations of two light bridges in active region NOAA 8990 is presented. The observations were recorded with the La Palma Stokes Polarimeter attached to the Swedish Vacuum Solar Telescope. The stratification over the solar atmosphere of different physical parameters is retrieved from these data using the Stokes inversion based on response functions (SIR). Our results confirm previous observations of features such as the decrease in magnetic field strength and the increase in inclination in the light bridges. We also confirm a temperature increase in these structures with respect to the surrounding umbrae. The maps of the magnetic field strength and of the orientation of the magnetic field vector indicate the presence of a canopy structure above the light bridges. We derive the vertical component of electric current density (Jz) from the configuration of the magnetic field. The increased temperature found in the upper layers is studied in the context of the proposed canopy topology and could also explain the recently observed chromospheric heating processes found above light bridges.

The thermal and magnetic structure of umbral dots from the inversion of high-resolution full stokes observations

This paper presents the analysis of high-resolution Stokes observations of eight different umbral dots in a sunspot. The spectra were recorded with the La Palma Stokes Polarimeter, attached to the Swedish Vacuum Solar Telescope. The observed line profiles have been inverted to yield the height stratifications of temperature, magnetic field, and line-of-sight velocity, as well as their respective Wilson depressions. We report on systematic differences in the properties of umbral dots with respect to the nearby umbra, including small upflows (~100 m s-1), higher temperatures (~1 kK), and weaker fields (~500 G) with more horizontal orientations (~10°). The field weakening is strongly correlated with the Wilson depression, suggesting that it may be due to an opacity effect (as one is looking at higher layers). The inclination excess, on the other hand, is real and cannot be ascribed to formation height issues. The results obtained from our semiempirical modeling are discussed within the context of the currently existing scenarios for the subsurface structure of sunspots. The observational signatures revealed by our analysis fit well within both the spaghetti and the monolithic models.

EVOLUTION OF PHYSICAL CHARACTERISTICS OF UMBRAL DOTS AND PENUMBRAL GRAINS

A time series of full-Stokes spectropolarimetric observations of the sunspot NOAA 10944, acquired with HINODE/SOT in 2007 February, is analyzed. The data were inverted using the code SIR into a series of 34 maps covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength, and inclination in two different ranges of optical depths corresponding to the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots (CUDs and PUDs) and penumbral grains (PGs) was traced. While CUDs do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows of 400 m s–1 and a more horizontal magnetic field are detected in the low photospheric layers of PUDs. PGs have even stronger upflows and magnetic field inclination in the low photosphere than PUDs. The absolute values of these parameters decrease when PGs evolve into PUDs. It seems that PGs and PUDs are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, CUDs appear in regions with a stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination.

Fine structure in sunspots - IV. Penumbral grains in speckle reconstructed images

The properties of penumbral grains (PGs) in a large regular sunspot are studied from a 70 min sequence of G -band images acquired on 20 September 1999 at the Dutch Open Telescope, La Palma. The frames were processed using the speckle masking algorithm, resulting in an almost diffraction-limited time series (30 s cadence), basically free of atmospheric distortions. Applying feature tracking to a movie of 140 frames yields proper motions, intensities, and lifetimes for a set of 1058 PGs with lifetimes longer than 10 min. About 54% of the PGs move toward the umbra and 46% toward the photosphere. The inward-moving PGs are located mostly in the inner penumbra (up to 0.6 of the distance from the umbra to the photosphere). Their average lifetime and median speed are 50 min and 0.52 km s -1 . Most of the outward-moving PGs are observed in the outer penumbra and their average lifetime and median speed are 31 min and 0.75 km s -1 . These measurements confirm the previous results published by Sobotka et al. ([CITE]).

Morphology and evolution of umbral dots and their substructures

Substructures - dark lanes and tails - of umbral dots (UDs) were predicted by numerical simulations of magnetoconvection. We analyse a 6 h 23 min time series of broadband images of a large umbra in the active region NOAA 10634, acquired with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A 43 min part of this series was reconstructed with the MFBD method, reaching a spatial resolution of 0.14. We measure brightness, size, lifetime, and horizontal velocities of various umbral structures. Most (90 %) of UDs and bright point-like features in faint LBs split and merge, and their median lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is considered as the end of their life. Both UDs and features in faint LBs that do not split or merge are clearly smaller (0.15) than the average size (0.17) of all features. Horizontal motions of umbral bright small-scale features are directed either into the umbra or along faint LBs with mean horizontal velocities of 0.34 km/s. Features faster than 0.4 km/s appear mostly at the periphery of the umbra. The intensity of dark lanes, measured in four bright central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs. The width of dark lanes is probably less than the resolution limit 0.14. The characteristic time of substructure changes of UDs is ~4 min. We observe narrow (0.14) bright and dark filaments connected with PUDs. Usually one dark and two bright filaments form a 0.4 wide tail attached to one PUD, resembling a short dark-cored penumbral filament. Our results indicate the similarity between PUDs and PGs located at the tips of bright penumbral filaments. The features seen in numerical MHD simulations are consistent with our observations of dark lanes in CUDs and tails attached to PUDs.

Fine structure and dynamics in a light bridge inside a solar pore

A photometric analysis of the sub-structure of a granular light bridge in a large solar pore is performed. The data consist of a 66 min time series of white light images (A = 5425 A ′ 50 A) of an active region NOAA 7886 obtained at the Swedish Vacuum Solar Telescope on La Palma, Canary Islands. The light bridge can be resolved into an assembly of small grains embedded in a diffuse background with an intensity of about 85% of the mean photospheric intensity (I p h o t ). Following the temporal evolution of these sub-structures in their irregular motions inside the light bridge, proper motions with velocities up to 1.5 km s - 1 can be detected. Their lifetime distribution shows a maximum at 5 min and a second peak at approximately 20 min. The origin and the decay of these sub-structures is very similar to those of granules, i.e. fragmentation, merging and spontaneous origination from, and dissolution into, the background can be observed. Some of them are able to escape from the light bridge into the umbra where they cannot be distinguished from adjacent umbral dots. Generally, this study presents evidence that the observed phenomenon represents convective motions.

Photometry of umbral dots

Until now, the size of umbral dots has been considered to be below the resolution limit of large solar telescopes. We analyze observations of two sunspots and two pores, acquired in September 2003 with the new 1-m Swedish Solar Telescope, La Palma. White-light images with a resolution better than 0. �� 15 were taken simultaneously in blue (451 nm) and red (602 nm) wavelength bands. They were corrected for scattered light and restored for the instrumental profile of the telescope. Intensities, diameters and positions of umbral dots were measured in aligned pairs of images in the blue and red wavelength band. We find that observed intensities of umbral dots are correlated with local intensities of umbral background. On average, UDs are by about 1000 K hotter than the coolest area in the umbra and by 500–1000 K cooler than the undisturbed photosphere. Individual UDs may reach or exceed the average photospheric brightness and temperature. Histograms of observed diameters peak at 0. 23 (170 km). This indicates that the majority of umbral dots are spatially resolved with the 1-m telescope. The mean nearest-neighbour distance between umbral dots is 0. �� 4 and their average observed filling factor is 9%. The method of two-colour photometry is discussed and applied to obtain average “true” intensities and diameters. About 50% of umbral dots are brighter than the quiet photosphere and the average “true” diameter of umbral dots is 100 km. However, the latter results might be influenced by systematic errors of the method.

Properties of sunspot moats derived from horizontal motions

Context: Sunspots in late phases of evolution are usually surrounded by annular moats, regions where systematic horizontal flows are observed to be directed radially away from the spot. These flows are considered to be a manifestation of the sub-photospheric convection. Aims: The characteristics of moats are derived at two different heights in the solar atmosphere from horizontal motions around sunspots of different sizes, shapes, and phases of evolution. We also study the temporal evolution of moats. Methods: Local correlation tracking is applied to approximately 70-min long time series of white-light and 1600 A images, acquired by the satellite TRACE, to analyse the horizontal motions of photospheric granules and C IV emission structures in the vicinity of 32 sunspots. Moat regions are defined by means of radially-oriented, outward velocities. Results: Relations between sunspot types and the occurrence, areas, and horizontal velocities of moats in the photosphere and transition region are described. Moats do not show substantial changes during the period of about 12 h. Observed asymmetries in moat shapes and velocities are related to the height in the atmosphere, to sunspot age, and to proper motion. It is suggested that the sub-photospheric convective flows around sunspots may be influenced by the spots proper motion through the convection zone.

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.