Magnetohydrodynamic modes in the Earth’s outer core under the Quasi-Geostrophic approximation

Abstract:

The geomagnetic field is known to show oscillations and variability on a wide range of timescales. These are believed to be the observable consequence of the rich dynamics taking place in the outer core of the Earth, where fluid motions driven mainly by thermochemical convection generate and constantly modify the geomagnetic field.

On short timescales it has been demonstrated that, because of the overwhelming importance of rotation compared to other forces acting on the fluid, motions are organized in columns and are essentially 2-D. This approximation is called Quasi-Geostrophy (QG) and allows us to build models that strongly simplify our description of the core dynamics.

The QG approximation has been applied in previous works to geomagnetic data inversion to retrieve the flows at the surface of the core. But these techniques provide only a static picture of the core flows and do not offer insights into the magnetic field below the CMB.

We are developing a dynamical numerical model based on the QG approximation. This is the first step toward a Data Assimilation system, in which we will constrain the evolution of the numerical model with real geomagnetic observations to obtain a dynamical picture of the magnetic and velocity fields below the CMB. As this is an inverse problem, it is essential to first characterize the forward model. We will illustrate the normal modes sustained by our system and compare them with known results. In particular we retrieve the splitting between “fast” hydrodynamic modes and “slow” magneto-hydrodynamic modes. The latter are one of the best candidates to explain the westward drift, a well known feature of the geomagnetic secular variation.