The rotation rate of solar active and ephemeral regions -- II. Temporal variations of the rotation rates

TL;DR

This study uses magnetic field maps to analyze the rotation rates of solar active regions, finding that their rotation remains mostly constant during decay and that variations during emergence are not systematic.

Contribution

It introduces a magnetic field map-based method to study active region rotation rates, challenging previous findings of systematic acceleration or deceleration.

Findings

Active regions show no systematic change in rotation rate during decay.

Rotation rate varies during emergence, with possible acceleration or deceleration.

Rotation rate remains nearly constant during the decay phase of active regions.

Abstract

Systematic studies of the rotation rate of sunspot groups using white-light images yield controversial results on the variations of the rotation rate: sunspot groups were found to either accelerate or decelerate systematically. This disagreement might be related to shortcomings of the method used to probe the rotation rate of sunspot groups. In contrast to previous works, in this study we use magnetic field maps to analyse the variations of the rotation rate of active regions. We found that an active region may exhibit either acceleration or deceleration during the emergence while the rotation rate remains almost unchanged during decay. Hence, we suppose that there is no systematic geometrical inclination to the radial direction of the apex of the subsurface magnetic flux loop forming an active region. A thorough comparison of the rotation rate of unipolar and bi/multipolar active regions revealed no significant changes in the rotation rate of decaying active regions. In contrast to previous works, we presume the rotation rate to keep constant (within the expected uncertainties) during the evolution of an active region after emergence.