Surface and interior meridional circulation in the Sun

Abstract

Solar meridional circulation is an axisymmetric flow system, extending from the equator to the poles ($$\sim $$ ∼ 20 m/s at the surface, $$\approx $$ ≈ 1% of the mean solar rotation rate), plunging inwards and subsequently completing the circuit in the interior through an equatorward return flow and a radially outward flow back up to the surface. This article reviews the profound role that meridional circulation plays in maintaining global dynamics and regulating large-scale solar magnetism. Because it is relatively weak in comparison to differential rotation ($$\sim $$ ∼ 300 m/s, $$\approx $$ ≈ 7% of the mean solar rotation rate) and owing to numerous systematical errors, accurate surface measurements were only first made in 1978 and initial inferences of interior meridional circulation were obtained using helioseismology two decades later. However, systematical biases have made it very challenging to reliably recover flow in the deep interior. Despite numerous advances that have served to improve the accuracy of inferences, the location of the return flow and the full extent of the circulation are still open problems. This article follows the historical developments and summarises contemporary advances that have led to modern inferences of surface and interior meridional flow.

Magnetic Flux in the Sun Emerges Unaffected by Supergranular-scale Surface Flows

“…Global-scale background flows on the Sun, such as differential rotation (Howe 2009) and meridional circulation (Hanasoge 2022), can induce systematic variations in the measured local flow velocities. This is undesirable, as we are only interested in the flow structures associated with AR emergence.…”

Mean field models of flux transport dynamo and meridional circulation in the Sun and stars

“…The extra condition of the thermo-rotational balance cannot be satisfied near the boundary. Thin boundary layers form where the balance (Rajaguru and Antia, 2015;Gizon et al, 2020;Hanasoge, 2022). An unsettled issue is how deep the equatorward meridional counter flow penetrates within the solar interior.…”

“…Magnetic field can also affect the meridional flow indirectly by modifying the differential temperature (Spruit, 2003;Hanasoge, 2022) or differential rotation.…”

Exploring the Solar Poles: The Last Great Frontier of the Sun

“…In particular, Dopplergram data would allow us to apply helioseismic methods (Christensen-Dalsgaard, 2002;Gizon and Birch, 2005) to infer solar internal structure and dynamics. Meridional circulationwhich plays a critical role in setting the timescale and amplitude of large-scale solar dynamo action and magnetism (Rempel 2006;Hanasoge, 2022) is rather poorly constrained at the poles. Significant improvements in flow measurements in the vicinity of the polar cap, including the polar extent of the meridional circulation are possible with the magnetic feature tracking technique (Mahajan et al 2021) applied to observations that do not suffer from projection effects.…”