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

TL;DR

The paper investigates solar-cycle variations of meridional and zonal flows within the Sun's near-surface shear layer (NSSL) by combining long-term global helioseismic inversions of rotation with local time-distance inversions of meridional flows. It demonstrates a robust correlation between residual meridional flows, $\,U_{ heta}$, and the radial gradient of rotation, $\delta(\partial\log\Omega/\partial\log r)$, driven primarily by the Coriolis force acting on flows toward active latitudes. A depth-dependent sign change occurs around $0.97\,R_{\\odot}$, where near-surface inflows give way to downflow-connected outflows, while zonal-flow changes remain modest and are not driven by these near-surface inflows. The results, validated by agreement between global and local helioseismology techniques and by 3D TD inversions around active regions, constrain the dynamical coupling between meridional circulation, rotational shear, and magnetic activity in the NSSL, with implications for solar dynamo theories.

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 ($>$5$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.

Figures (5)

• Figure 1: Changes (Cycle 24 subtracted) in dimensionless radial gradient of rotation rate, $\delta(\partial~{\rm{log}}~\Omega/\partial~{\rm{log}}~r)$, from HMI global helioseismic rotation inversions (top panels) and those in the meridional flow, $\delta U_{\theta}$, (middle and bottom panels from HMI and GONG, respectively) as a function of latitude and time at 0.99 $R_{\odot}$ (left) and at 0.95 $R_{\odot}$ (right). Sunspot locations are overplotted as black dots. The two vertical dashed lines mark Cycle 24 maximum (2014) and minimum (2020).
• Figure 2: Latitude profiles of changes in meridional flow ($\delta U_{\theta}$, in black) and zonal flow ($\delta U_{\phi}$, in pink), averaged over one year around the Cycle 24 maximum, for depths 0.99 $R_{\odot}$ ( solid) and 0.95 $R_{\odot}$ ( dashed) in the top panel. The same in the radial gradient of rotation, $\delta(\partial~{\rm{log}}~\Omega / \partial~{\rm{log}}~r)$, along with the longitudinally averaged unsigned magnetic field (right y-axis), is shown in the lower panel. The errorbars shown represent errors estimated in the inversion method see Section \ref{['subsec: results_1']} for details.
• Figure 3: Connections between the time vs depth profiles of changes in meridional flow ($\delta U_{\theta}$) and that in the radial gradient of rotation [$\delta(\partial~{\rm{log}}~\Omega / \partial~{\rm{log}}~r)$]. The top and middle panels show $\delta U_{\theta}$ for the northern and southern latitudes (20$^{o}$,25$^{o}$ and 30$^{o}$), respectively. The north - south symmetric component of changes in $\delta(\partial log \Omega / \partial log r)$ are shown in the bottom row for latitudes (10$^{o}$, 15$^{o}$, and 20$^{o}$). The two vertical dashed lines in each panel mark Cycle 24 maximum (2014) and minimum (2020), and the horizontal dotted lines mark the depth 0.97 $R_{\odot}$. All the results here are from the HMI data.
• Figure 4: Local time-distance helioseismic inversions for flows around a large active region NOAA: 12192 (S12W08) observed on 2014 October 23. The flows, plotted as arrows here, are averaged over the near-surface depths of 0 - 3 Mm ($\approx$ 0.99 $R_{\odot}$, left panel) and over 13 - 21 Mm ($\approx$ 0.97 $R_{\odot}$, right panel). The background color image is the LOS magnetic field map of the bipolar spot region.
• Figure 5: Sketch showing the Coriolis force mediated average flow structures around active regions in the northern hemisphere. The labels depict the signs of residuals in meridional flows, $\delta U_{\theta}$, and that in resulting residual rotational shear for the two depths, 0.99 and 0.95 $R_{\odot}$, which mark the radial boundaries of the NSSL.