Torsional Alfven Waves Excited Inside the Earth's Core Because of Modulations by the Solar Cycle of the Electrical Currents Flowing in the Magnetosphere

HR: 1340h
AN: MR43A-0871 [Abstracts]
TI: Torsional Alfven Waves Excited Inside the Earth's Core Because of Modulations by the Solar Cycle of the Electrical Currents Flowing in the Magnetosphere
AU: LＨaut, G
EM: Gedeon.Legaut@obs.ujf-grenoble.fr
AF: LGIT, University of Grenoble BP 53, Grenoble, 38041 France
AU: * Jault, D
EM: Dominique.Jault@obs.ujf-grenoble.fr
AF: LGIT, University of Grenoble BP 53, Grenoble, 38041 France
AB: Torsional Alfven waves consist of rigid rotations of geostrophic cylinders coupled together by the magnetic field permeating the Earth's core. Fluctuations in the rate of rotation of the solid mantle arise from the coupling between the torsional waves and the rotational motion of the mantle. Depending on the electrical conductivity of the Earth's mantle, more or less intense electrical currents are induced in a thin layer at the top of the Earth's core by variations of the magnetic field of external origin. We find that oscillations of the geostrophic cylinders near the equator of the Earth's core are readily excited by these time varying electrical currents. That sets off torsional waves, which propagate inward and dissipate through magnetic friction with the solid inner core. These waves take part to the fluid motions at the core surface and interact with the main magnetic field. As a result, there is an interior magnetic field associated to any time varying external magnetic field with a long enough period. We find that the ratio between the magnitudes of respectively these internal and external magnetic fields increases rapidly with the period. The exact values depend on the electrical conductivities of both the core and the mantle and on the intensity of the main magnetic field inside the core. With current estimates of these parameters, the ratio between the strength of the magnetic field generated by the torsional oscillations and of the strength of the inducing external magnetic field can be much larger than 1 at the period of the solar cycle. We suggest that part of the secular variation may have an external origin. Our mechanism may successfully explain the rapid succession, every ten years or so, of magnetic "jerks" from 1969 until now.
DE: 1510 Dynamo theories
DE: 1555 Time variations--diurnal to secular
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting