Oscillations in sunspots near the solar limb and the influence of seeing effects
Abstract
Oscillations in sunspots near the solar limb were observed by measuring the variation of their geometric distance to a reference point (the limb) on time series of spectrograms. Observations carried out with this recently introduced technique are quite sensitive to seeing variations. Therefore we present a simple model to estimate the influence of seeing effects on the observations quantitatively. It turns out, that under poor seeing conditions the seeing-induced variations of the spot's position are of the same order of magnitude as the expected displacements due to oscillations. The strong influence of the seeing is clearly demonstrated for a series, which was obtained at the Capri station under unfavourable seeing conditions, in the frequency range where solar oscillations are expected.
A second sunspot was observed simultaneously under good seeing conditions in Hα, Hβ, the nearby Fe I 4860 Å line and in the continuum with the VTT equipment on Tenerife. As a result we find evidence for unusual non-photo spheric umbral oscillations in the range of 0.7±0.5 mHz and 3.5±0.5 mHz.- Publication:
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Astronomy and Astrophysics
- Pub Date:
- September 1993
- Bibcode:
- 1993A&A...276..227F
- Keywords:
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- Sun: sunspots;
- atmospheric effects;
- Sun: oscillations