On the sunspot transition region

Abstract

The EUV line emission and relative line-of-sight velocity in the transition region between the chromosphere and corona of 36 sunspot regions are investigated, based on observations with the Coronal Diagnostic Spectrometer - CDS and the Solar Ultraviolet Measurements of Emitted Radiation - SUMER on the Solar and Heliospheric Observatory - SOHO. The most prominent features in the transition-region intensity maps are the sunspot plumes. In the temperature range between log T=5.2 and log T=5.6 we find that 29 of the 36 sunspots contain one or two sunspot plumes. The relative line-of-sight velocity in sunspot plumes is high and directed into the Sun in the transition region, for 19 of the sunspots the maximum velocity exceeds 25 km s⁻¹. The velocity increases with increasing temperature, reaches a maximum close to log T=5.5 and then decreases abruptly.