Events

Using Solar Dynamics Observatory/Helioseismic and Magnetic Imager ring-diagram analysis, we infer the rotation rate Ω(r,θ,t) in the near-surface shear layer (NSSL) over depths of 1–17 Mm and quantify both its temporal variability and the dimensionless radial shear d ln Ω / d ln r. The inferred mean rotation profile increases inward and matches global-mode results where the depth ranges overlap, while revealing measurable north–south differences. We show that the time variation of the rotation rate can be determined even without subtracting the time-averaged rotation rate from each epoch. The cumulative zonal displacement inferred from the residual flows exhibits a pronounced high-latitude hemispheric asymmetry and varies on solar-cycle timescales. At 75o it shows an apparent temporal association with the polar magnetic field. Cross-correlation analysis between cumulative displacement and magnetic activity indices show a hemispheric dependence at mid-to-high latitudes suggestive of differences in cycle timing between hemispheres. We further find that the NSSL radial shear exhibits a three-region structure featuring an enhanced-shear middle layer; the largest residual shear variations occur in the shallow layers not accessible to global-mode helioseismology. Parametrizing the enhanced-shear layer by the depth of maximum shear, amplitude, and width, we show all three are strongly correlated with a magnetic activity index. Higher activity corresponds to a shallower, stronger, and modestly narrower layer. This behavior is consistent with expectations that strong toroidal fields can enhance near-surface rotational shear (Kitchatinov 2016), and with inferences of a near-surface toroidal-field concentration near the radius of maximum shear (Baldner et al. 2009). The strengthening and upward shift of the strong-shear layer toward solar-cycle maximum suggests a corresponding cycle dependence in the depth of the strong toroidal field.