Solar-cycle variations in meridional flows and rotational shear within the Sun's near-surface shear layer

TL;DR

This study investigates how solar-cycle variations influence meridional flows and rotational shear in the Sun's near-surface layer, revealing the role of the Coriolis force and flow patterns related to active regions.

Contribution

It provides new insights into the spatio-temporal variations of flows in the NSSL and their connection to solar activity, highlighting the influence of the Coriolis force on these dynamics.

Findings

Coriolis force affects large-scale flow variations during the solar cycle.

Near-surface inflows towards active regions form a local circulation pattern.

Changes in rotation gradient are linked to flow variations and solar activity.

Abstract

Using solar-cycle long helioseismic measurements of meridional and zonal flows in the near-surface shear layer (NSSL) of the Sun, we study their spatio-temporal variations and connections to active regions. We find that near-surface inflows towards active latitudes are part of a local circulation with an outflow away from them at depths around 0.97 R, which is also the location where the deviations in the radial gradient of rotation change sign. These results, together with opposite-signed changes over latitude and depth in the above quantities observed during the solar minimum period, point to the action of the Coriolis force on large-scale flows as the primary cause of changes in the rotation gradient within the NSSL. We also find that such Coriolis force-mediated changes in near-surface flows towards active latitudes only marginally change the amplitude of zonal flow and hence are not likely to be its driving force. Our measurements typically achieve a high signal-to-noise ratio ($>$5$\sigma$) for near-surface flows but can drop to 3$\sigma$ near the base (0.95 R) of the NSSL. Close agreements between the depth profiles of changes in rotation gradient and in meridional flows measured from quite different global and local helioseismic techniques, respectively, show that the results are not dependent on the analysis techniques.