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Fine structure in a dark umbra
- The University of Akron Press
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Modern Solar Facilities – Advanced Solar Science, 205–208 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c Universitätsverlag Göttingen 2007 Fine structure in a dark umbra M. Sobotka1,* and K. G. Puschmann2 1 Astronomical Institute AS CR, Ondřejov, Czech Republic 2 Institut für Astrophysik, Göttingen, Germany * Email: msobotka@asu.cas.cz Abstract. An excellent-quality time series of images of a large dark umbra of the leading sunspot of NOAA 10634 was acquired on 18 June 2004 with the 1-m Swedish Solar Telescope at La Palma, simultaneously in blue, red, and G-band channels. The temporal and spatial resolutions are 20 s and 0. 14. A 2-hour long series of the red continuum images is analysed, showing the faintest umbral fine structures. In addition to umbral dots, often clustered to more stable “granules” or aligned to short chains, we observe large, low-intensity elongated structures with dark central channels, resembling extremely faint light bridges. At the periphery of the umbra, bright umbral dots move inwards, showing a similarity to penumbral grains. Kinematic properties of umbral fine structures are studied. 1 Introduction Sunspot umbrae, when observed under high spatial resolution and sufficient signal-to-noise ratio, show inhomogeneities in brightness. Chevalier (1916) detected a small-scale granularlike pattern in the umbra in his collection of sunspot photographs obtained under a resolution of 0. 7–1 . The existence of “umbral granulation” was reported later by several observers (Thiessen 1950; Bray & Loughhead 1964; Bumba et al. 1975). Bumba & Suda (1980) claimed that the spatial distribution of “granules” inside the umbra is identical with that in the photosphere. The term “umbral dots” (UDs) was introduced by Danielson (1964). In the set of photographs from the balloon-borne experiment Stratoscope he detected very small, bright point-like features. The spatial distribution of UDs was different from the photospheric granulation pattern. It is accepted now that the “umbral granules” correspond to unresolved groups and clusters of UDs observed with moderate spatial resolution. Referring to observations with the 91-cm telescope at Kitt Peak, Livingston (1991) claimed that the umbra has a radial filamentary structure with dark “voids” (dark nuclei) free of UDs between bright filaments. So, UDs might be only seeing-induced artifacts. However, the existence of UDs was confirmed by many posterior observations and UDs have been considered as typical representatives of the bright umbral component, embedded in the dark, “diffuse” umbral background with smoothly varying intensity (Sobotka 1999). 2 Observations and data processing The large leading sunspot of NOAA 10634 was observed on 18 June 2004 from 07:43 to 15:30 UT with the 1-m Swedish Solar Telescope (SST, Scharmer et al. 2003) equipped with 206 M. Sobotka and K. G. Puschmann: Fine structure in a dark umbra adaptive optics. The spot, located near disk centre (N13, E6), was in the phase of growth. The images were acquired in a frame-selection mode (selection interval 20 s), simultaneously in three wavelength bands: blue (4507.5±4.6) Å, red (6020.0±13.0) Å, and G band (4308.6± 5.8) Å. Exposure times were 11–14 ms and the pixel size corresponded to 0. 0405. After dark- and flatfield corrections, the stray light was eliminated following Martı́nez Pillet (1992). Parameters of the scattering were determined for each wavelength from the shapes of photometric profiles across the solar limb. The level of stray light, originating mostly in the instrument, was 8.5 % in blue, 6.5 % in red, and 8.7 % in G band. The deconvolution of the instrumental profile of the diffraction-limited 1-m telescope and the noise filtering was carried out simultaneously, applying Wiener filters with noise suppression starting at 0. 11 (blue), 0. 14 (red), and 0. 13 (G band). Regarding the correction of wavefront aberrations done by the adaptive optics, these values characterise the spatial resolution in the best frames. No other restoring techniques were applied. The image rotation was compensated and the frames were aligned and de-stretched. Finally , a subsonic filter with a cutoff at 4 km s−1 was applied to the series of images. For further analyses we selected the best part of the time series taken in the red channel from 12:15 to 14:12 UT, containing 350 frames. The field of view was reduced to 20. 25×20. 25 (500×500 pixels), covering most of the umbra. For the purpose of visualisation and feature tracking, it is good to eliminate large...