Magnetic damping of the translational oscillations of the inner core

Abstract

SUMMARY We calculate the magnetic perturbation that occurs near the inner core boundary when the conducting materials on either side of the boundary are displaced by translational oscillations of the inner core. The associated Lorentz forces and ohmic losses cause changes in the period and damping of the oscillations which may be detectable in gravity measurements. Estimates of these changes depend on the average value of the radial main field Br near the inner core boundary. The value of Br also affects the character of the magnetic disturbance in the fluid core. For Br 0.002T the perturbation is transmitted by Alfven waves. The most observable effects are associated with the damping of the oscillations. Typical values of the damping factor Q vary widely with Br, but may be as low as 2000. Since Q is extremely sensitive to the main magnetic field we suggest that observations of the inner core oscillations might be used to infer the average strength of the field near the inner core boundary. To facilitate the interpretation of the gravity measurements, we present analytical expressions for the effect of a magnetic field on the period and Q of inner core oscillations.