Equatorially trapped waves in Earth's core

Short-period fluctuations in geomagnetic acceleration are detected in recent satellite observations. A major component of this signal is confined to the equatorial region, suggesting the presence of equatorial waves, but the precise nature of these waves is not known. We explore the possibility that these waves arise from an interplay of magnetic, Archimedes and Coriolis forces in a stratified layer at the top of the core (sometimes called MAC waves). We adopt a beta-plane approximation and show that low-frequency MAC waves are not trapped near the equator when the root-mean-square (rms) radial magnetic field is constant over the surface of the core-mantle boundary. However, equatorial trapped MAC waves emerge when the rms radial magnetic field increases towards the poles. Further confinement of MAC waves occurs when we account for the leading-order effects of spherical geometry. The resulting MAC waves propagate to the east with phase velocities that depend strongly on the thickness of the stratified layer. Waves with periods less than 10 yr are predicted when the layer thickness is less than 30 km. These waves have low quality factors, Q approximate to 1, which means that they propagate only a few thousand kilometers before being dissipated by ohmic losses. Evidence for eastward and westward propagating disturbances in the observations may reflect a superposition of wave propagation and forced motion by an excitation source. Separation of the source from the wave propagation may be possible if the source is due mainly to westward drifting plumes in the equatorial region.