Analysis of SOHO/MDI and TRACE Observations of Sunspot Torsional   Oscillation in AR10421
  Rotation of the leading sunspot of active region NOAA 10421 was investigated using magnetograms and Dopplergrams from the MDI instrument of the Solar and Heliospheric Observatory (SOHO), and white-light images from the Transition Region and Coronal Explorer (TRACE). The vertical, radial, and azimuthal axisymmetrical components of both magnetic and velocity field vectors were reconstructed for the sunspot umbra and penumbra. All three components of both vectors in the umbra and penumbra show torsional oscillations with the same rotational period of about 3.8 days. The TRACE white-light data also show that the sunspot umbra and penumbra are torsionally rotating with the same period. Possible mechanisms of sunspot torsional motions are discussed. 
O.S. Gopasyuk and A.G. Kosovichev
astro-ph.SR