ANGULAR VELOCITIES OF SUNSPOT-GROUPS AND SOLAR PHOTOSPHERIC ROTATION

From the analysis of the Greenwich Photoheliographic Results sunspot-groups data from 1874 to 1976, it has been evidenced that the sunspot-groups angular velocity is not invariant with time during the first 2.5 10(6) s of their life, showing a non-random negative acceleration. More precisely, 2 days old sunspot-groups have, at all the latitude strips, angular velocities which are on average, 0.3 degrees/day higher than recurrent sunspots. Moreover, the angular velocity difference between sunspot-groups of age less than 8.6 10(5) s and recurrent sunspots decreases with time, and after 2 rotations, the angular velocity is almost equal to the plasma velocity. On the basis of these results, the problem of the determination of the solar photospheric rotation has been re-examined. We have obtained two main results: a) the differences in the rotation curves obtained by other authors who used type and area as discriminating parameters are due to the parameter time only; b) the best tracers for the measurement of the photospheric rotation rate are the oldest sunspot-groups. The implications of a higher angular velocity of young tracers on the subphotospheric layer rotation are discussed.