Torsional oscillations in sunspots derived from SOHO/MDI data

High-resolution (0.6 arc sec/pixel) MDI data maps with two single sunspots seen in brightness, line-of-sight velocity and magnetic field are analyzed. Using the assumption of the cylindrical symmetry, the radial, tangential and vertical components of the velocities are calculated for a number of distances from the center of the sunspot. to its penumbral outer limit. Time variations of these components show significant power in a period ran-e near 2-5 days and in the well-known 5-min hand. The power in the 5-n-min region would be associated with interaction between acoustic oscillation and the sunspot structure. Analysis of the velocity components vs. distance from the center of spot shows no substantial temporal variations for both radial and vertical ones. Nevertheless. the tangential velocity component is changed significantly and reverses its sign to the opposite one. The most important velocity changes are located near the outer limit of the spot's umbra. This phenomenon call be interpreted as a torsional oscillations of the sunspot. An explanation of these oscillations feature is presented and discussed.