Statistical weights and selective height corrections in the determination of the solar rotation velocity

Abstract

Observations of the Sun performed at 37 GHz with the 14-m radio telescope of the Metsahovi Radio Observatory were analyzed. Rotation velocities were determined, tracing Low Temperature Regions (LTRs) in the years 1979–1980, 1981–1982, 1987–1988, and 1989–1991. Statistical weights were ascribed to the determined rotation velocities of LTRs, according to the number of tracing days. Measured changes of the rotation velocity during the solar activity cycle, as well as a north–south rotation asymmetry, are discussed. The results obtained with and without the statistical weights procedure are compared, and it was found that the statistical significance of the solar differential rotation parameters' changes is higher when the statistical weights procedure is applied. A selective application of the height correction on LTR's positions has not removed the cycle-related changes nor the north–south asymmetry of the solar rotation measured tracing LTRs. So, projection effects cannot explain these changes. The differential rotation of LTRs is more rigid than the differential rotation obtained tracing magnetic features and measuring Doppler shifts, which can be explained by the association rate of the LTRs' positions with rigidly rotating `pivot points'. The observed cycle-related changes and the north–south asymmetry of the rotation velocity of LTRs are consistent with the cycle-related changes and the north–south asymmetry of the association rate between LTRs and pivot points.