OBSERVATIONS OF SUNSPOT UMBRAL OSCILLATIONS

The solar vacuum telescopes VTT and GCT at Tenerife have been used to obtain high-resolution two-dimensional spectro-polarimetric observatio ns of oscillations in the photospheric layers of sunspots. At the GCT the area of the sunspot has been scanned by shifting the spectrograph slit; at the VTT a Fabry-Perot interferometer has been applied to get narrow-band filtergrams directly and to scan through the line profile. The spectra of velocity oscillations show the known features of close ly packed power peaks in bands of periods around 3 min (strengthened) and 5 min (weakened with respect to the quiet Sun). In the same freque ncy bands the more reliable VTT data show significant oscillations of the magnetic field strength as well, which could not be attributed to disturbing influences. Maximum power of both velocity and magnetic osc illations and a strong correlation between them, in the 3-min band in particular, is found to occur in those parts of the umbra where the ma gnetic lines of force are parallel to the line of sight. The oscillati ons are characterized by a marked spatial fine structure and a non-sta tionary behaviour.