Radiation belt electron acceleration by ULF wave drift resonance: Simulation of 1997 and 1998 storms

Abstract

Pc 5 ULF waves are seen concurrently with the rise in radiation belt fluxes associated with CME-magnetic cloud events. Four such geomagnetic storm periods in 1997 and 1998, 10-11 January and 14-15 May, 1997; 1-4 May and 26-28 August, 1998, have been simulated with a 3D global MHD code driven by Ll-measured solar wind parameters. The field output has been used to advance electron guiding center trajectories in the equatorial plane. The time series has also been analyzed for ULF wave mode structure. Toroidal field line resonances with low azimuthal mode number and frequencies commensurate with the electron drift period are identified in the radial electric field component, along with enhanced power in the poloidal (azimuthal) electric field component. The simulated time scale for inward radial transport of electrons in the several hundred keV to MeV energy range, from geosynchronous orbit to L=3-4, varies with the level of ULF wave power and overall energy input to the magnetosphere. Of the four events studied, the May 1998 storm period was most geoeffective and the January 1997 least so, in terms of simulated radial transport and energization of electrons. This transport rate is consistent with the level of ULF waves excited in the simulations, and the proposed drift resonant acceleration mechanism.