Evidence for MAC waves in the core and implications for the thermal state

Abstract:

Earth's liquid core hosts a diverse set of waves with periods ranging from days to thousands of years. One class of waves with periods of several decades are known to arise from an interplay between magnetic, Archimedes and Coriolis forces. These so-called MAC waves are thought to be relevant for interpreting historical fluctuations in the geomagnetic field. We show that MAC waves provide a good description of time-dependent zonal flow at the top of the core. The same collection of waves also offers a simple explanation for observed fluctuations in the dipole field. Both of these predictions require a 140-kilometer-thick stratified layer at the top of the core with a buoyancy frequency comparable to Earth's rotation rate. We extend these predictions to include changes in the length of day and find that MAC waves can account for about half of the observed fluctuation at decadal periods. Larger fluctuations are possible when electromagnetic stresses couple MAC waves to flow in the interior of the core. In fact, an idealized model for the coupled system over estimates the length-of-day fluctuations, possibly reflecting limitations in the idealized model. The waves do not discriminate between thermal and compositional stratification at the top of the core, but the properties of the layer are compatible with thermal stratification, implying a core heat flow of 13 TW relative to an adiabatic heat flow of 15 TW.