Temporal variations of the solar rotation rate at high latitudes

Abstract

Frequency splitting coefficients from Global Oscillation Network Group and Michelson Doppler Imager observations covering the period 1995-2001 are used to study temporal variations in the solar rotation rate at high latitudes. The torsional oscillation pattern in the Sun is known to penetrate to a depth of about 0.1 R_⊙ with alternate bands of faster and slower rotating plasma. At lower latitudes, the bands move toward the equator with time. At higher latitudes, however, the bands appear to move toward the poles. This is similar to the observed poleward movement of large-scale magnetic fields at high latitudes. This also supports theoretical results of poleward-moving bands at high latitudes in some mean field dynamo models. The polar rotation rate is found to decrease between 1995 and 1999, after which it has started increasing.