Active-region Modulation of Subsurface Meridional Flows and Magnetic Flux Transport on the Sun

TL;DR

This study uses 14 years of helioseismology data to show that active-region magnetic fields modulate subsurface meridional flows, influencing the solar magnetic flux transport and polar field evolution across solar cycles.

Contribution

It reveals how depth-dependent, activity-modulated meridional flows in the near-surface shear layer impact magnetic flux transport and polar field buildup, advancing understanding of the solar dynamo process.

Findings

Polar field buildup is linked to diverging plasma outflows from active latitudes.

Stronger southern outflows during cycle 24 accelerated flux transport, causing earlier polar field peaks in the south.

Global flux transport patterns are consistent across multiple solar cycles, influenced by deeper meridional flow modulation.

Abstract

Using time-distance helioseismology applied to 14-years of SDO/HMI observations spanning solar cycle 24 and rising phase of cycle 25, we present evidence that meridional flows in the lower half of the near-surface shear layer (NSSL), modulated by active-region magnetic fields, play a central role in the episodic global transport of magnetic flux. In particular, polar field buildup is tightly linked to plasma outflows diverging from active latitudes within the deeper NSSL. The magnitude and timing of hemispheric polar field evolution are regulated by depth-dependent meridional flow, including its cross-equatorial component, responding to active-region flux asymmetries. During cycle 24 maximum, stronger southern outflows accelerated flux transport, causing the southern polar field to peak nearly four years before the northern. Global magnetic flux transport patterns in the previous three solar cycles (21, 22, and 23) show broad consistency with the deeper meridional flow modulation inferred in cycles 24 and 25. These results identify activity-dependent flow variations in deeper layers of the NSSL as a dynamically significant component of the Babcock-Leighton process that governs the generation and hemispheric asymmetry of global dipole field.