Limb Darkening and Solar Rotation--II: Observed Darkening in 1966

Abstract

Spectrophotometric measures of surface brightness at 6263 Å for diametrically opposite limbs of the Sun confirm that the poles are at least five per cent brighter than the equatorial limb. Solutions by Radau integration for the source function and computation of photospheric models give temperature as a function of linear height for a number of latitudes. The currents due to the resulting meridional temperature-gradient occur in a photospheric Ekmanlayer and are of the same order of magnitude as those found from velocities measured at Oxford in 1953 and 1960. These thermal currents are stable and are maintained against viscous loss by heat absorbed in the photosphere. To these currents is probably due the observed decrease of angular velocity towards the poles of the Sun. The origin of polar heating is however as yet unknown.