Solar equatorial plasma rotation: a comparison of different spectroscopic measurements

We present solar equatorial rotation velocities measured with two different spectral lines (Fe I 557.6 nm and Ni I 676.8 nm) and two different spectro meters at the German Vacuum Tower Telescope (VTT) on Tenerife. The 'classic al' sidereal solar equatorial rotation velocity of about 2000 m/s has been confirmed. The results are compared with those from velocity data of the Mi chelson Doppler Imager (MDI) onboard the Solar Heliospheric Observatory (SO HO) obtained on 10 June 1996 and 27 May 1999. From both data sets of MDI, a rotation velocity about 100 to 200 m/s below the 'classical: value cited a bove was found. Possible explanations of this discrepancy are discussed.