Abstract:
Using time–distance helioseismology applied to 14 yr of Solar Dynamics Observatory/Helioseismic and
Magnetic Imager observations spanning solar cycle 24 and the 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
4 yr 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 the
global dipole field.
