Rapid geomagnetic variations and stable stratification at the top of Earth's core

Abstract

Probing the possible presence and physical properties of a stably stratified layer atop Earth's core is crucial to better determine the past history and heat budget of the planet. This has previously been done by ascribing a variety of interannual to decadal geomagnetic variations to hydromagnetic waves internal to the layer. This study presents the first self-consistent simulation of the stratified layer dynamics in interplay with the underlying core convection, in physical conditions matching those of Earth's core. Magneto-Archimedes-Coriolis waves of decadal periods appear in stratified layers deeper than a few tens of kilometers and with Brunt-Väisälä frequency matching the rotation rate of the planet. However, the level at which core convection excites these waves is generally insufficient to account for observed geomagnetic variations in this period range. Strong stratification is furthermore deleterious to a number of observed features that unstratified models are successful at reproducing. Fluid flow at the core surface decouples from the interior and becomes strongly dissimilar to geomagnetic inferences. Magnetic jerks and their corresponding near-equatorial, rapidly alternating magnetic acceleration patterns also disappear, because the supporting interannual magneto-Coriolis waves are impeded by the stratified layer. This negative impact on the reproduction of the observed rapid geomagnetic variations limits the possible extent of a stable top layer to the first few tens of kilometers beneath Earth's core surface.