Understanding solar torsional oscillations from global dynamo models

TL;DR

This paper uses global MHD simulations to investigate the origin of solar torsional oscillations, highlighting the role of magnetic tension in the tachocline in driving these migratory zonal flows associated with the solar cycle.

Contribution

It demonstrates that magnetic tension in the tachocline is a key driver of torsional oscillations, providing new insights into their origin and the modulation of surface flows.

Findings

Magnetic tension in the tachocline influences zonal flow variations.

Torque from magnetic tension causes pole acceleration and equator slowdown.

Surface flow changes propagate from the tachocline through the convection zone.

Abstract

The phenomenon of solar "torsional oscillations" (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed-up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.