Abstract
We obtained coordinated observations of the large sunspot in NOAA Region 8539 on 1999 May 9 and 13 with the Very Large Array and three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory satellite. The EUV observations reveal a plume in the sunspot umbra on both observing dates. The plume appears brightest in emission lines formed at temperatures between 1.6 × 105 and 5.0 × 105 K. Radio emission from the sunspot umbra is dominated by thermal gyroemission from the plume, which accounts for radio brightness temperatures <1 × 106 K in the umbra on both dates, as well as umbral brightness temperature depressions in the 4.535 and 8.065 GHz observations on May 13. A compact 14.665 GHz source persists near the umbra/penumbra boundary during our observing period, indicating a long-lived, compact flux tube with coronal magnetic field strength of at least 1748 G. It occurs in a portion of the sunspot that appears very dark in EUV emission.
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