CORE FLOW INSTABILITIES AND GEOMAGNETIC STORMS DURING REVERSALS - THESTEENS MOUNTAIN IMPULSIVE FIELD VARIATIONS REVISITED

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 vari ations imply extremely large variation rates for the ambient field, fo ur orders of magnitude above the present-day secular variation. Two di fferent mechanisms in the liquid outer core (namely turbulence and ine rtial waves) that may produce such variations are investigated. Energe tic considerations indicate that the observed impulsive variations are unlikely to be of internal (core) origin. On the other hand, it is sh own that during reversals, when the dipolar field is reduced by a fact or ten, magnetic storm perturbations at the Earth's surface are enhanc ed 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 variati ons recorded at Steens Mountain is shown to be consistent with the eff ect 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 intern al field during the impulsive change.