SOLAR SURFACE VELOCITY-FIELDS DETERMINED FROM SMALL MAGNETIC FEATURES

We describe a method for the analysis of magnetic data taken daily at the Vacuum Telescope at Kitt Peak. In this technique, accurate position differences of very small magnetic features on the solar surface outside active regions are determined from one day to the next by a cross-correlation analysis. In order to minimize systematic errors, a number of corrections are applied to the data for effects originating in the instrument and in the Earth's atmosphere. The resulting maps of solar latitude vs central meridian distance are cross-correlated from one day to the next to determine daily motions in longitude and latitude. Some examples of rotation and meridional motion results are presented. For the months of May 1988 and October-November 1987, we find rotation coefficients A = 2.894 +/- 0.0011, B = -0.428 +/- 0.070, and C = -0.370 +/- 0.077 in mu-rad s-1 from the expansion-omega = A + B sin2-phi + C sin4-phi, where phi is the latitude. The differential rotation curve for this interval is essentially flat within 20 deg of the equator in these intervals. For the same intervals we find a poleward meridional motion a = 16.0 +/- 2.8 m sec-1 from the relation upsilon = a sin-phi, where upsilon is the line-of-sight velocity.