Variability in Sunspot Associated Microwave Emission: Umbral Oscillations?
Session 68 -- Solar Activity
Oral presentation, Thursday, January 13, 2:15-3:45, Salons A/B Room (Crystal City Marriott)

## [68.08] Variability in Sunspot Associated Microwave Emission: Umbral Oscillations?

N. Gopalswamy, E. J. Schmahl, M.R. Kundu (University of Maryland)

We report on microwave observations of sunspot associated emission that shows variability over a time scale of minutes. To our knowledge, this is the first time such rapid variability has been observed in microwave radiation from a sunspot. These observations were obtained by the Very Large Array (VLA) on April 24, 1992 at 2cm. The radio emission from the sunspot umbra was in the form of several compact sources with a size less than 4 arcsec. The time evolution of the peak flux of these sources showed significant time varaiations which were sometimes periodic. The period of these ocillations was approximately 3 min, similar to that of intensity and Doppler shift oscillations observed in optically thin, transition region lines such as C IV (1548.19 \AA) in sunspot umbrae. There were also morphological changes in the extended sunspot emission over which the compact sources were superposed. We also observed the appearance of new compact sources within the umbra where there was reduced emission before. The brightness temperatures of these compact sources were in the range $(1-5)\times 10^5 K$. We explore possible interpretations of the time variability.