Satellite magnetic data reveal interannual waves in Earth's core

The Earth's magnetic field displays variations on a broad range of time scales, from years to hundreds of millions of years. The last two decades of global and continuous satellite geomagnetic field monitoring have considerably enriched the knowledge on the rapid physical processes taking place in the Earth's outer core. Identification of axisymmetric torsional Alfven waves with subdecadal periods from observatory and satellite data has given access to an averaged intensity of the magnetic field in the Earth's core interior. A significant part of the rapid signal, however, resides in nonaxisymmetric motions. Their origin has remained elusive, as previous studies of magnetohydrodynamic waves in the Earth's core mainly focused on their possible signature on centennial time scales. Here, we identify nonaxisymmetric wavelike patterns in the equatorial region of the core surface from the observed geomagnetic variations. These wavelike features have large spatial scales, interannual periods in the vicinity of 7 y, amplitudes reaching 3 km/y, and coherent westward drift at phase speeds of about 1,500 km/y. We interpret and model these flows as the signature of Magneto-Coriolis (MC) eigenmodes. Their identification offers a way to probe the cylindrical radial component of the magnetic field inside Earth's core. It follows from our work that there is no need for a stratified layer at the top of the core to account for the rapid geomagnetic field changes.