CCD-Doppler measurements of solar differential rotation

Abstract

Using the AT1 CCD camera at the Echelle spectrograph of the GCT at Tenerife, solar Doppler rotation measurements in the photospheric lines Fe I 6301.5 A and 6302.5 A and in the chromospheric line Na-D2 5890.0 A have been made. The line shifts measured at different heliographic latitudes around the limb were corrected for observer motion and converted into sidereal rotation rates. At the equator the observed chromospheric rotation rate is about 8 % larger than the photospheric rate, and the average observed Doppler rotation rate is not very much different from the mean rotation rates deduced from ‘all’ published tracer works and ‘all’ published Doppler works. Near the poles (where tracer methods rely on extrapolation) both the chromospheric and the photospheric rotation rate are slightly smaller than the ‘all’ Doppler rate and are considerably smaller than the extrapolated ‘all’ tracer rate. If ‘all’ previous measurements of solar rotation are taken into account, a surface rotation law with lower error bounds than previously possible can be derived.