Magnetostrophic MRI in the Earth's outer core

Ludovic Petitdemange^1,2, Emmanuel Dormy^1,3, Steven A. Balbus^1,2

¹ Laboratoire de Radioastronomie, Département de Physique, Ecole Normale Supérieure, Paris, France
² Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Observatoire de Paris, CNRS/UMR8112, Paris, France
³ Institut de Physique du Globe de Paris, CNRS/UMR7154, Paris, France

GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L15305, doi:10.1029/2008GL034395, 2008

Received 18 April 2008; accepted 26 June 2008; published 15 August 2008.

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.