Observations of sunspot transition region oscillations

Oscillations with a period of 3 minutes are observed in the transition regi on of six sunspots with the Solar and Heliospheric Observatory - SOHO joint observing programme for velocity fields in sunspot regions. Observations o f the transition region lines O V lambda 629 and N V lambda lambda 1238, 12 42 with the SUMER instrument show significant differences in the amplitude of the 3-minute oscillations from one sunspot to another, both in intensity and line-of-sight velocity. In four sunspots the central part of the umbra is observed. Two of these sunspots show coincidence between the maxima in peak line intensity and velocity directed towards the observer, as is expec ted for an upward-propagating acoustic wave. The two other sunspots show la rge oscillation amplitudes and a difference of 25 degrees between maxima in intensity and blue shift. The possible effect of partial wave reflection o n the observed phase relation is discussed. For one sunspot only a part of the umbra, close to the penumbra, was observed and the observations show a difference of 50 degrees between maxima in intensity and blueshift. For the smallest sunspot the observations are found to be contaminated by contribu tions from an area without oscillations. Observed oscillations in line widt h are small, but probably significant in two sunspots. The observations of NOAA 8378 allow us to compare simultaneous recordings of the oscillations i n the chromospheric Si II lambda 1260 line with the oscillations in the tra nsition region lines. We question the suggestion by Fludra (1999) that the sunspot transition region oscillations are a typical feature of the sunspot plumes.