The 1969 geomagnetic impulse and spin-up of the Earth's liquid core

We relax the steady motions assumption in inversions for the velocity field at the core surface, allowing the flow to change instantaneously at the jerk epoch, then allowing one, and, finally, two spherical harmonic flow coefficients to change continuously with time. The period 1965-1975 covering the 1969 magnetic jerk epoch has been investigated using spherical harmonic models of the geomagnetic field and secular variation. An instantaneous change in the flow cannot account for the jerk signal. An otherwise steady flow with one coefficient changing with time can produce a jerk signal in the SV at the surface of the Earth, but the statistics show that the fit to the input models is only modestly improved. We find evidence of spin-up of the core, in the increase of the axisymmetric flow component after the jerk, from a series of flows each assumed steady over 5 yrs. An inversion with two coefficients dependent on time further demonstrates the acceleration of the flow and also shows a lag of the solid body flow component behind an equatorial acceleration, suggesting that the spherical geometry of the core-mantle boundary has a strong influence on the core dynamics. An estimate of the spin-up time of 5 yrs gives an approximate eddy viscosity for the core of 7 m(2) s(-1). Neglecting magnetic fields and buoyancy, the results suggest that hydrodynamics of spin-up in a spherical container with a turbulent Ekman boundary layer occurs in the same manner as with a laminar boundary layer. Toroidal fields at the core surface of the order of 1 mT and weak stratification 10(-9) K/km attenuate the action of boundary layers. However, poloidal magnetic field of strength 1 mT and stratification of the order 10(-2) K/km can produce two-dimensional flow in an electrically insulating cylindrical container. If the container is conducting, the interaction of the Hartmann currents in the fluid and container produces two-dimensional flow on a spin-up time scale of 5 yrs involving a weakly conducting whole mantle, with mean conductivity of 0.1 S m(-1). (C) 1997 Elsevier Science B.V.