Sunspot plumes and flow channels

It is well known that sunspots are dark. This statement is not correct in t he sunspot atmosphere between the chromosphere and the corona, where sunspo ts often are brighter than their surroundings. The brightest feature in the sunspot transition region is called a sunspot plume. Not all sunspots cont ain a plume. We find that 20 out of 21 sunspots show a plume when one magne tic polarity dominates the sunspot region out to a distance of 50" from the sunspot. Most sunspots show downflows that exceed 25 km s(-1) in the sunsp ot plumes at temperatures close to 250 000 K. This downflow is not maintain ed by inflow from the corona, but by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. We sug gest that this inflow is a necessary requirement for the sunspot plume to o ccur and present a working hypothesis for the origin of sunspot plumes. Thi s paper is the first thorough spectral analysis of sunspot plumes. It is ba sed on simultaneous observations of ten or six EUV emission lines in 42 sun spot regions with the Coronal Diagnostic Spectrometer - CDS on the Solar an d Heliospheric Observatory - SOHO. The line profiles are studied in detail with another SOHO instrument, the Solar Ultraviolet Measurements of Emitted Radiation - SUMER.