Evolution of Subsurface Zonal and Meridional Flows in Solar Cycle 24 from Helioseismological Data

TL;DR

This study analyzes 10 years of helioseismological data to reveal how subsurface zonal and meridional flows in the Sun evolve with the solar cycle, showing patterns linked to magnetic activity and serving as dynamo indicators.

Contribution

It provides the first detailed analysis of subsurface flow variations over an entire solar cycle using helioseismology, highlighting their extended cycle behavior and potential as activity precursors.

Findings

Zonal flow belts migrate from high latitudes to the equator over the cycle.

Zonal velocity increases correlate with sunspot numbers and magnetic field strength.

Growth of zonal velocities in 2018 signals the start of Cycle 25.

Abstract

The results of determinations of the azimuthal and meridional velocities by time-distance helioseismology from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) from May 2010 to September 2020 at latitudes from -60{\deg} to +60{\deg} and depths to about 19 Mm below the photosphere are used to analyze spatiotemporal variations of the solar differential rotation and meridional circulation. The pattern of torsional oscillations, or latitudinal belts of alternating `fast' and `slow' zonal flows migrating from high latitudes towards the equator, is found to extend in the time--latitude diagrams over the whole time interval. The oscillation period is comparable with a doubled solar-activity-cycle and can be described as an extended solar cycle. The zonal-velocity variations are related to the solar-activity level, the local-velocity increases corresponding to the sunspot-number increases and being localized at latitudes where the strongest magnetic fields are recorded. The dramatic growth of the zonal velocities in 2018 appears to be a precursor of the beginning of activity Cycle 25. The strong symmetrization of the zonal-velocity field by 2020 can be considered another precursor. The general pattern of poleward meridional flows is modulated by latitudinal variations that are similar to the extended-solar-cycle behavior of the zonal flows. During the activity maximum, these variations are superposed with a higher harmonic corresponding to meridional flows converging to the spot-formation latitudes. Our results indicate that variations of both the zonal and meridional flows exhibit the extended solar-cycle behavior, which is an intrinsic feature of the solar dynamo.