The Role of the Magnetorotational Instability in the Sun

TL;DR

This study investigates the magnetorotational instability (MRI) in the Sun, calculating growth rates and identifying unstable modes throughout the solar interior, revealing MRI's potential role in solar magnetic field dynamics.

Contribution

The paper derives a dispersion relation for nonaxisymmetric MRI modes in the Sun and applies it using helioseismic data to identify where MRI is significant.

Findings

MRI modes are present near the tachocline at certain latitudes.

Nonaxisymmetric MRI modes grow faster than axisymmetric ones.

The implied magnetic field strength from MRI saturation aligns with dynamo estimates.

Abstract

We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group (GONG) helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic (MHD) shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes theta less than 53 degrees. Nonaxisymmetric MRI modes grow faster than the corresponding axisymmetric modes; for some poloidal magnetic fields, the nonaxisymmetric MRI growth rates are similar to the angular rotation frequency Omega, while axisymmetric modes are stabilized. We briefly discuss the saturation of the field produced by MRI modes, finding that the implied field at the base of the convective zone in the Sun is comparable to that derived based on dynamos active in the tachocline and that the saturation of field resulting from the MRI may be of importance even in the upper convection zone.