Hemispheric Magnetic Asymmetry and Cross-Equatorial Circulation Cells within the Sun's Near-Surface Shear Layer

TL;DR

The study tackles how cross-equatorial meridional flows in the Sun’s near-surface shear layer are modulated by hemispheric magnetic asymmetry. It uses 14 years of time-distance helioseismology with HMI and GONG data, masking active regions and computing a mass-conserving flow field to map depth-dependent patterns. The key finding is a depth-dependent circulation: upper-layer inflows toward the more active hemisphere and deeper outflows below $0.97R_{\odot}$ form cross-equatorial circulation cells with lifetimes ~2 years, while surface flux plumes move opposite to the surface flow due to deeper advection. Removing active regions reduces but does not eliminate the cross-equatorial flows, suggesting a substantial role for deeper or decayed flux in sustaining the observed dynamics, with important implications for magnetic flux transport and solar-cycle evolution.

Abstract

Using time-distance helioseismic measurements of meridional flow in the near-surface shear layer over a period of 14 years, starting from May 2010, we probe the depth structure and evolution of its cross-equatorial part. We confirm that the hemispheric magnetic asymmetry determines the amplitude and direction of such flows. Additionally, we find that these flows turn over and change direction at depths below 0.97R, forming circulation cells with lifetimes dictated again by the hemispheric magnetic imbalance, which is dominated by the occurrences of large sunspots. We also examine connections between cross-equatorial magnetic flux plumes and the flows, and discuss their implications for the equatorial flux cancellation/submergence and the poleward transport of flux.

Figures (8)

• Figure 1: Time-depth profiles of meridional flow, $U_{\theta}$, averaged over $\pm 5^{o}$ across the equator, are shown in the top two panels that compare measurements from GONG and HMI. Positive (negative) values represent northward (southward) flows, and the depth range covers the whole of the near-surface shear layer. The bottom panel shows the magnetic butterfly diagram derived from the HMI LOS magnetic field, denoted simply as B, for the same period. The overplotted black and pink curves, with right y-axes, are the time-variations N-S asymmetry in the absolute hemispheric magnetic field averaged over active latitudes ($\pm 20^{o}$), $< \vert$B$\vert_{N}>$ - $<\vert$B$\vert_{S}>$, and N-S average of signed magnetic field over the equator ($\pm 5^{o}$), respectively. Both curves have been smoothed using a 6-month running average.
• Figure 2: Cuts across different depths of the 2D time-depth profile of cross-equatorial flows shown in Figure \ref{['fig:1']} for GONG and HMI: blue (0.995 $R_{\odot}$), green (0.98 $R_{\odot}$), magenta (0.965 $R_{\odot}$), and red (0.95 $R_{\odot}$). The estimated error for each depth is marked within the panels. The dashed black curve in both panels is the same as in Figure \ref{['fig:1']}, except that here we have applied a 12-month running average, and it corresponds to the right Y-axis.
• Figure 3: Time - latitude profile of near-surface (0.99 $R_{\odot}$) meridional flow (over $\pm$10$^{\circ}$ across the equator) is compared with that of sunspot locations and sizes. The time period chosen (2011 - 2017) is that of the active phase of Solar Cycle 24. The middle panel displays the locations and sizes of all sunspots, with the colorbar restricted to 200 millionths of a hemisphere ($\mu$HS), while the lower panel shows that of large sunspots with an area greater than 1000 millionths of a solar hemisphere. The NOAA sunspot group numbers of the large spots are listed within the panel. The two vertical dashed lines mark the time of change in flow direction across the equator.
• Figure 4: Examples depicting how sunspot regions and their surroundings are removed in flow maps before longitudinally averaging them to test the extent of contributions from flows around active regions to the cross-equatorial flows. This test is performed on the local 3D (latitude, longitude and depth) time-distance helioseismic flow inversions that map the upper 20 Mm of the convection zone. The two panels correspond to dates when episodes of large cross-equatorial flows were measured.
• Figure 5: Baseline quiet-Sun meridional flow profile at a depth of 3.5 Mm determined by averaging over a 1-year period during Cycle 24 minimum (mid-2019 to mid-2020).