Tracing sunspot groups to determine angular momentum transfer on the Sun

Abstract

In this paper, our goal is to investigate Reynolds stress and to check whether it is plausible that this is responsible for angular momentum transfer towards the solar equator. We have also analysed meridional velocity, rotation velocity residuals and correlation between the velocities. We have used the position measurements of sunspot groups from the Greenwich Photographic Result and the Solar Observing Optical Network/United States Air Force/National Oceanic and Atmospheric Administration data bases, covering the period 1878–2011. In order to calculate the velocities, we used the daily motion of sunspot groups. The sample was also limited to ±58° in the central meridian distance in order to avoid solar limb effects. We have mainly investigated velocity patterns depending on the solar cycle phase and latitude. We have found that the meridional motion of sunspot groups is towards the centre of activity from all available latitudes and in all phases of the solar cycle. The range of meridional velocities is ±10 m s−1. Horizontal Reynolds stress is negative at all available latitudes and indicates that there is a minimum value (q ≈ −3000 m2 s−2) located at b ≈ ±30°. In our convention, this means that angular momentum is transported towards the solar equator, in agreement with the observed rotational profile of the Sun.