Core flow instabilities and geomagnetic storms during reversals: The Steens Mountain impulsive field variations revisited

Abstract

The origin of the large variations in magnetization direction recorded in two different lava flows at Steens Mountain, Oregon, is discussed. The time constant for lava-flow cooling and the amplitude of the variations imply extremely large variation rates for the ambient field, four orders of magnitude above the present-day secular variation. Two different mechanisms in the liquid outer core (namely turbulence and inertial waves) that may produce such variations are investigated. Energetic considerations indicate that the observed impulsive variations are unlikely to be of internal (core) origin. On the other hand, it is shown that during reversals, when the dipolar field is reduced by a factor ten, magnetic storm perturbations at the Earth's surface are enhanced by a factor two with respect to their present amplitude. It follows that intense magnetic storms can significantly contribute to ambient field changes during reversals. The amplitude of the impulsive variations recorded at Steens Mountain is shown to be consistent with the effect of a strong magnetic storm occurring during cooling of a flow. The direction of the variations is also shown to be consistent with this mechanism if the dipolar field contributes significantly to the internal field during the impulsive change.