Signatures of High-Latitude Waves in Observations of Geomagnetic Acceleration

Models for the second time-derivative of the geomagnetic field reveal prominent activity at high latitudes. Alternating patches of positive and negative geomagnetic acceleration propagate to the west at speeds that exceed nominal fluid velocities in the core. We show that waves are a viable interpretation of these observations. Magnetic Rossby waves produce a high-latitude response with suitable phase velocities. However, the spatial complexity of the prediction is not compatible with the observations. Our preferred interpretation involves zonal MAC waves. These waves can account for the observed geomagnetic field when a stratified layer exists at the top of the core. The required layer has a thickness in excess of 100 km and a buoyancy frequency comparable to the rotation frequency. We anticipate a gradual reduction in the phase velocity over time, leading to a future change in the propagation direction. Plain Language Summary Earth's magnetic field is generated by motion of liquid metal in the outer core. Variations in Earth's magnetic field from 1999 to present-day have been observed by the satellites Swarm, CryoSat-2, CHAMP, SAC-C, and Orsted. These observations show a train of alternating geomagnetic acceleration and deceleration at high latitude (similar to 66 degrees N) which move to the west. These features travel faster than fluid velocities in the outer core, suggesting an origin due to fast wave propagation. In this work, we explore two viable types of waves: magnetic Rossby waves and zonal MAC waves. Both waves would propagate within a stratified layer at the top of the core. The required layer has a thickness greater than 100 km. Zonal MAC waves are found to better explain the observations. This interpretation allows us to predict that the high-latitude features will slow down, then change direction and propagate east. Key Points Geomagnetic acceleration at high latitudes is attributed to waves in Earth's core Waves with suitable periods and phase velocities require fluid stratification The proposed stratification is compatible with a thermal origin

Automated summary

The acceleration of Earth's magnetic field at high latitudes is attributed to waves in the core, with potential involvement of magnetic Rossby waves and zonal MAC waves. Observations suggest that these waves may propagate within a stratified layer at the top of the core, and the predicted stratification will lead to a slowing down and change in propagation direction of high-latitude features.