A HIGH-RESOLUTION STUDY OF THE STRUCTURE OF SUNSPOT LIGHT BRIDGES ANDABNORMAL GRANULATION

Strong light bridges (SLBs) represent an abrupt change in the physical conditions of sunspot umbrae. They divide the umbra into separate uni ts (fragments, umbral cores) and very often show a granular structure. A photometric and spectroscopic study of SLBs, and also of abnormal g ranulation (AG) in a region of pores, is presented. Slit-jaw images (b andpass 5425 +/- 50 angstrom) and spectra (line Fe I 5434.5 angstrom), with a spatial resolution of 0.3'', were acquired during the period 1 991 July 1-10 at the Swedish Solar Telescope at the Observatorio del R oque de los Muchachos (La Palma). The power spectra of intensity fluct uations in the slit-jaw images, were used as the primary diagnostic to ol, complemented by the analysis of line profiles. The following resul ts were obtained. 1. The structures present in SLBs and AG are general ly smaller than the granules in the quiet photosphere. The typical siz e of SLB granules is 1.2'' (in quiet granulation, 1.5''). In AG, the d istribution of power is more complex and is characterized by peaks cor responding to scales of 2.5'', 1.3'', and 0.64''. 2. SLB and AG power spectra show an excess of power (compared to quiet granulation) at sca les of 0.5''. This power enhancement reflects the presence of small br ight grains, clearly visible in SLBs and AG, with a mean nearest neigh bor distance of 0.5''. 3. Two of these small bright grains, together w ith a dark lane between them, were resolved spectroscopically in a SLB . The line shifts and bisector shapes suggest a convective origin of t hese structures. 4. The SLB power spectra in the log P/log k scale ind icate the presence of a Kolmogorov turbulent cascade in the structures between 1.7'' and 0.64''. However, the small bright grains do not per tain to the turbulent cascade. They are particular phenomena, probably with a convective origin. The power spectrum of AG does not give any indication of the presence of a turbulent cascade. 5. Small bright fea tures (umbral dots or bright grains) can be found in umbral cores, lig ht bridges, and AG regions as well. Their mean nearest neighbor distan ce, in the observed areas, decreases with increasing brightness and de creasing magnetic field strength. We thus propose that they are a mani festation of altered convection, which is modulated and maintained by the magnetic field.