Variations in Differential Rotation and Meridional Flow within the Sun's Surface Shear Layer 1996-2022

TL;DR

This study analyzes solar surface flow variations over 26 years, revealing cycle-dependent changes in differential rotation and meridional flow, with implications for understanding solar magnetic activity and internal dynamics.

Contribution

It provides a comprehensive analysis of the temporal and latitudinal variations in the Sun's surface flows across multiple solar cycles using magnetogram data.

Findings

Equatorial torsional oscillations drift equatorward during cycle decline.

Polar rotation varies with cycle phase, faster at maxima.

Meridional flow slows during cycle rise and speeds during decline.

Abstract

We measure differential rotation and meridional flow in the Sun's surface shear layer by tracking the motions of the magnetic network seen in magnetograms from SOHO/MDI and SDO/HMI over solar cycles 23, 24, and the start of 25 (1996-2022). We examine the axisymmetric flows derived from 15-24 daily measurements averaged over individual 27-day Carrington rotations. Variations in the differential rotation include the equatorial torsional oscillation - cyclonic flows centered on the active latitudes with slower flows on the poleward sides of the active latitudes and faster flows equatorward. The fast flow band starts at $\sim$45$^\circ$ latitude during the declining phase of the previous cycle and drifts equatorward, terminating at the equator at about the time of cycle minimum. Variations in the differential rotation also include a polar oscillation above 45$^\circ$ with faster rotation at cycle maxima and slower rotation at cycle minima. The equatorial variations were stronger in cycle 24 than in cycle 23 but the polar variations were weaker. Variations in the meridional flow include a slowing of the poleward flow in the active latitudes during cycle rise and maximum and a speeding up of the poleward flow during cycle decline and minimum. The slowing in the active latitudes was less pronounced in cycle 24 than in cycle 23. Polar countercells (equatorward flow) extend from the poles down to $\sim$60$^\circ$ latitude from time to time (1996-2000 and 2016-2022 in the south and 2001-2011 and 2017-2022 in the north). Both axisymmetric flows vary in strength with depth. The rotation rate increases inward while the meridional flow weakens inward.