Sobotka, Michal; Bonet, Jose A.; Vazquez, Manuel
Bibliographical reference
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 426, no. 1, p. 404-413
Advertised on:
5
1994
Journal
Citations
58
Refereed citations
56
Description
Strong light bridges (SLBs) represent an abrupt change in the physical
conditions of sunspot umbrae. They divide the umbra into separate units
(fragments, umbral cores) and very often show a granular structure. A
photometric and spectroscopic study of SLBs, and also of abnormal
granulation (AG) in a region of pores, is presented. Slit-jaw images
(bandpass 5425 +/- 50 A) and spectra (line Fe I 5434.5 A), with a
spatial resolution of 0.3 sec, were acquired during the period 1991 July
1-10 at the Swedish Solar Telescope at the Observatorio del Roque de los
Muchachos (La Palma). The power spectra of intensity fluctuations in the
slit-jaw images, were used as the primary diagnostic tool, complemented
by the analysis of line profiles. The following results were obtained.
1. The structures present in SLBs and AG are generally smaller than the
granules in the quiet photosphere. The typical size of SLB granules is
1.2 sec (in quiet granulation, 1.5 sec). In AG, the distribution of
power is more complex and is characterized by peaks corresponding to
scales of 2.5 sec, 1.3 sec, and 0.64 sec. 2. SLB and AG power spectra
show an excess of power (compared to quiet granulation) at scales of 0.5
sec. This power enhancement reflects the presence of small bright
grains, clearly visible in SLBs and AG, with a mean nearest neighbor
distance of 0.5 sec. 3. Two of these small bright grains, together with
a dark lane between them, were resolved spectroscopically in a SLB. The
line shifts and bisector shapes suggest a convective origin of these
structures. 4. The SLB power spectra in the log P/log k scale indicate
the presence of a Kolmogorov turbulent cascade in the structures between
1.7 sec and 0.64 sec. However, the small bright grains do not pertain 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 features (umbral
dots or bright grains) can be found in unbral cores, light bridges, and
AG regions as well.