VELOCITY OSCILLATIONS IN ACTIVE SUNSPOT GROUPS

Time series of two-dimensional spectra were taken with the Gottingen 2 D spectrometer at the VTT on Tenerife in 1996. They were investigated for Doppler velocities and velocity oscillations in small spots and po res of rapidly evolving sunspot groups. For the present measurements t he magnetically insensitive lines Fe I 557.6 nm and Fe I 709.0 nm were selected. Spots with penumbrae exhibit the Evershed effect. Some pore s seem to be connected with downflows, but the centres of the downflow s are somewhat displaced from their associated pores. The surroundings of the pores show red shifts relative to the whole field of view. The power in the 5-min range is reduced inside the spots and pores as wel l as in their immediate vicinity. This reduction inside the spots is i n agreement with former results. Outside the area of the spot group th e 5-min power has a patchy structure with a typical size of 5 are sec. For periods below 3.3 min, the behaviour of individual spots and pore s is different. Some spots show clearly enhanced power for these perio ds, and it remains high down to the Nyquist period at 1.5 min. The sma ll pores do not show enhanced three-minute oscillations compared with their vicinity. Inside one spot we find a ring of enhanced power in th e period range between 8 and 20 min corresponding to the time scales o f granular variations. This result could be an indication of a relatio n between spots and convection, but magneto-accoustic waves are also p ossible. The same ring exhibits also enhanced power for short periods.