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

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 ($>$5$σ$) for near-surface flows but can drop to 3$σ$ 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.

💡 Research Summary

This paper presents a comprehensive analysis of solar-cycle variations in large-scale flows within the Sun’s Near-Surface Shear Layer (NSSL), the region just below the photosphere where rotation rate increases rapidly with depth. By synergistically analyzing long-term helioseismic measurements of both meridional (north-south) and zonal (east-west) flows, the study investigates their spatiotemporal evolution and connection to magnetic active regions.

The core methodology involves computing residuals (δ) by subtracting the average over Solar Cycle 24 from time-series measurements. For zonal flows and the radial gradient of rotation, this uses global helioseismic inversions of frequency splittings from SDO/HMI data. For meridional flows, local time-distance helioseismology is applied to Dopplergrams from both HMI and GONG, with careful removal of systematic center-to-limb effects and magnetic region masking.

The key findings are multi-faceted:

1. Depth-Dependent Anti-Correlation: The study reveals a striking depth dependence in the changes. Near the surface (~0.99 solar radii, R⊙), inflows toward active latitudes are observed, coinciding with a steeper-than-average radial rotation gradient (negative δ(∂logΩ/∂logr)). In contrast, at the base of the NSSL (~0.95 R⊙), outflows away from active latitudes occur alongside a flatter-than-average rotation gradient (positive δ(∂logΩ/∂logr)). The sign reversal for both flow and rotation gradient changes happens around 0.97 R⊙.
2. Three-Dimensional Circulation Cells: This pattern indicates the existence of local, depth-dependent circulation cells associated with active regions: surface inflows are connected to subsurface outflows at greater depth, forming a vertical loop.
3. Dominant Role of the Coriolis Force: The strong spatial and temporal correlation between changes in the meridional flow (δUθ) and changes in the rotational shear (δ(∂logΩ/∂logr)) is interpreted as evidence for the primary role of the Coriolis force. The action of the Coriolis force on the observed large-scale meridional flow perturbations (-2Ω × δU) is identified as the likely main driver for redistributing angular momentum and thus modulating the rotation gradient within the NSSL. This mechanism is further supported by opposite-signed changes observed at higher latitudes during solar minimum.
4. Origin of Zonal Flows (Torsional Oscillations): The research finds that the Coriolis-force-mediated changes in meridional flows only marginally alter the amplitude of the zonal flow residuals (δUφ). This suggests that the local circulations around active regions are not the primary driver of the large-scale, global torsional oscillation pattern. The zonal flow’s origin likely lies in broader, deep-seated convective or magnetohydrodynamic processes.
5. Validation by Technique Independence: A significant strength of the study is the close agreement between depth profiles derived from two fundamentally different helioseismic techniques—global mode inversions for rotation and local time-distance analysis for flows. This agreement confirms that the results are robust and not artifacts of a specific analysis method.

In conclusion, the paper provides compelling observational evidence that solar-cycle variations in the Sun’s near-surface shear layer are orchestrated by the Coriolis force acting on meridional flows perturbed by magnetic activity. While these flow variations directly modulate the rotational shear, they appear insufficient to generate the global zonal flows, pointing to a more complex origin for the torsional oscillations. These findings impose critical constraints on models of solar interior dynamics and the solar dynamo.