The influence of a stably stratified layer on the Earth's outer core waves.

Seismic studies, mineral physics, thermal evolution models and geomagnetic observations offer conflicting evidence about the presence of a stably stratified layer at the top of the Earth's fluid outer core. Such a convectively stable layer could have a strong influence on the Earth's hydromagnetic waves, propagating underneath the core-mantle boundary (CMB) that are used to probe the outermost region of the core. Here we present numerical solutions for the eigenmodes in a neutrally stratified sphere permeated by a magnetic field with and without a top stable layer, allowing for fluid exchanges between the stable layer and the neutrally stratified bulk of the core and angular momentum exchanges across the CMB through viscous- and electromagnetic coupling. On interannual time-scales, we find torsional Alfvén waves that are only marginally affected by weak to moderate stratification strength in the outer layer. At decadal time-scales similarly weak stable layers promote the appearance of waves, that propagate primarily within the stable layer itself and resemble Magneto-Archimedes-Coriolis (MAC) waves, even though they interact with the adiabatic fluid core below.