Rotation Factor: Dynamics and Interaction of the Earth’s Core and Mantle

Abstract

The crystalline solid inner core is the most distant and enigmatic part of our planet, and along with the Earth’s crust is the smallest one. The inner core was discovered in 1936 and to date there have been observed a number of its anomalous and puzzling features—low rigidity and viscosity (comparing to other solids), high seismic attenuation, strong anisotropy and differential rotation. The inner core is isolated from the upper solid Earth shells by the liquid outer core with low viscosity, and hence it can rotate, oscillate, precess, vibrate and move out along the spin axis. About 25 years ago studies of generation, evolution and sustainment of the Earth’s magnetic field launched active investigations of the crystalline core. Estimate of IC differential rotation velocity with respect to mantle is important for explaining a number of geodynamical patterns and effects—e.g. the observed global distribution of seismicity spatially coordinated with critical latitudes. One of the hypotheses associates the distribution with variation in the Earth’s rotation velocity. In this report we analyse information published thus far on differential rotation of the Earth’s inner core and acknowledge multiple controversies. The differential rotation velocity estimated by body waves is between 0° and 3° per year; the Earth’s normal mode data yield the velocities between –2.5° and –0.8° per year, whereas the most probable estimates are from –0.2° to 0.2° per year. The inner core stationary rotation envisaged by geodynamo simulations is unlikely; it is rather non-stationary—with accelerations/decelerations induced by the Earth’s mantle.