Dynamo Confinement of a Radiatively Spreading Solar Tachocline Revealed by Self-consistent Global Simulations

TL;DR

This study demonstrates through self-consistent global simulations that a dynamo-generated magnetic field can confine the solar tachocline against radiative spreading, explaining its observed thinness.

Contribution

First to show a self-consistent simulation of a magnetically confined tachocline in a solar-like radiative spreading regime.

Findings

Magnetic confinement persists over multiple cycles.

A novel skin effect allows magnetic fields to penetrate the radiative zone.

Dynamo-generated Maxwell stress maintains tachocline thinness.

Abstract

The helioseismically observed solar tachocline is a thin internal boundary layer of shear that separates the rigidly-rotating solar radiative zone from the differentially-rotating convective zone and is believed to play a central role in the 22-year solar dynamo cycle. The observed thinness of the tachocline has long been a mystery, given the expected tendency of such shear to undergo radiative spreading. Radiative spreading is the process by which the meridional circulation and angular velocity burrow into a stably-stratified fluid owing to the mitigating effect of radiative thermal diffusion. A confinement mechanism is thus required to keep the tachocline so thin. In previous work using global dynamo simulations, we achieved a statistically-stationary confined tachocline where the confinement mechanism was derived from the Maxwell stress arising from a dynamo-generated nonaxisymmetric poloidal magnetic field. However, the parameters chosen meant that the tachocline was confined against viscous spread instead of radiative spread. Here, we show that the previously identified dynamo confinement mechanism still succeeds in a simulation that lies in the more solar-like radiative spreading regime. In particular, a nonaxisymmetric, quasicyclic dynamo develops in the convective zone and overshoot layer, penetrates into the radiative zone via a novel type of skin effect, and creates a Maxwell stress that confines the tachocline over many magnetic cycles. To the best of our knowledge, this is the first fully self-consistent rendering of a confined tachocline in a global numerical simulation in the parameter regime appropriate to the Sun.