Magnetostrophic MRI in the Earth's Outer Core

TL;DR

This paper demonstrates that a modified magnetorotational instability can occur in Earth's outer core due to weak differential rotation and magnetostrophic balance, potentially influencing core dynamics.

Contribution

It introduces a new MRI mechanism specific to Earth's core conditions, differing from classical astrophysical MRI applications.

Findings

MRI can develop in Earth's outer core with background shear.

The instability relies on magnetostrophic balance and weak differential rotation.

Growth occurs on geophysically relevant timescales.

Abstract

We show that a simple, modified version of the Magnetorotational Instability (MRI) can develop in the outer liquid core of the Earth, in the presence of a background shear. It requires either thermal wind, or a primary instability, such as convection, to drive a weak differential rotation within the core. The force balance in the Earth's core is very unlike classical astrophysical applications of the MRI (such as gaseous disks around stars). Here, the weak differential rotation in the Earth core yields an instability by its constructive interaction with the planet's much larger rotation rate. The resulting destabilising mechanism is just strong enough to counteract stabilizing resistive effects, and produce growth on geophysically interesting timescales. We give a simple physical explanation of the instability, and show that it relies on a force balance appropriate to the Earth's core, known as magnetostrophic balance.