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

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.

💡 Research Summary

The authors present a comprehensive 14‑year helioseismic investigation (May 2010 – April 2024) of meridional flows crossing the solar equator within the near‑surface shear layer (NSSL). Using time‑distance helioseismology on both space‑based HMI (SDO) and ground‑based GONG Doppler data, they retrieve the latitudinal flow component Uθ at depths from 0.995 R⊙ down to 0.94 R⊙, averaging over ±5° of the equator and applying a 12‑month running mean to suppress short‑term noise. Active regions stronger than 40 G are masked to avoid magnetic contamination, and the inversion is constrained by a stream‑function formulation that enforces mass continuity.

The key findings are threefold. First, the amplitude and sign of the near‑surface equatorial flow are tightly correlated with the hemispheric magnetic asymmetry, defined as B_asym = ⟨|B|⟩_N – ⟨|B|⟩_S, where ⟨|B|⟩ is the absolute line‑of‑sight field averaged over active latitudes (0°–20°). When the northern hemisphere dominates (B_asym > 0) the flow near the surface is directed northward; when the southern hemisphere dominates (B_asym < 0) the flow reverses southward. This relationship holds throughout Solar Cycle 24, covering the rising phase (2011‑2013), the maximum (2013‑2015), and the declining phase (2016‑2018). The flow magnitude reaches ≈8 m s⁻¹ and persists for roughly two‑year intervals, matching earlier reports of long‑lived cross‑equatorial streams.

Second, the authors discover that at a depth of about 0.97 R⊙ (≈20 Mm) the flow changes sign, establishing a pair of circulation cells that span the NSSL. In the upper layer the flow is directed toward the magnetically dominant hemisphere, while in the lower layer it returns in the opposite direction. This depth‑dependent reversal is evident in both HMI and GONG datasets and is statistically robust, with error estimates derived from 1 000 Monte‑Carlo perturbations of the travel‑time measurements. The cells exhibit a similar two‑year lifetime, suggesting that the magnetic asymmetry not only drives the surface flow but also governs the deeper return flow.

Third, the study links these circulation cells to cross‑equatorial magnetic flux plumes (B_eq), defined as the signed magnetic field averaged over the equatorial belt (±5°). The temporal evolution of B_eq aligns with the flow reversal epochs: during periods when the southern hemisphere is magnetically dominant, positive flux plumes (the leading polarity of the south) are advected northward, and vice versa. Because the surface inflow and the deep return flow have opposite directions, the authors argue that the plumes are not simply the product of diffusion or emergence but are actively transported by the deep return branch of the circulation cells. This interpretation reconciles earlier observations that flux plumes appear as sudden injections rather than diffusive spreads.

The relationship between large sunspot groups (area > 1000 µHS) and the flow is also examined. The authors find that the most massive sunspot groups tend to emerge 8–12 months after the onset of a cross‑equatorial flow episode, typically at latitudes far from the equator in the hemisphere toward which the surface flow is directed. This suggests that the flow physically redistributes magnetic flux, influencing where subsequent active regions appear. Moreover, the spatial clustering of sunspots mirrors the flow direction: when the flow is northward, southern‑hemisphere spots become more dispersed, while northern spots remain clustered, and the opposite pattern holds for southward flow. This behavior is consistent with earlier reports (Komm 2022) that cross‑equatorial streams preferentially transport flux from the more active hemisphere.

In summary, the paper demonstrates that hemispheric magnetic asymmetry is the primary driver of cross‑equatorial meridional flows in the NSSL, and that these flows are part of a depth‑dependent circulation system with a return branch below 0.97 R⊙. The deep return flow advects cross‑equatorial magnetic flux plumes opposite to the surface inflow, thereby playing a crucial role in flux cancellation at the equator, submergence of magnetic structures, and the poleward transport that reverses the Sun’s global magnetic field. These findings provide new constraints for flux‑transport dynamo models, emphasizing the need to incorporate asymmetric, time‑varying circulation cells rather than a static, hemispherically antisymmetric meridional circulation.