Reeves, G. D.,
Co-Authors: R. H. W. Friedel, M. G. Henderson, R. D. Belian, T. E. Cayton, M. M. Meier, D. N. Baker, X. Li, S. Kanekal, J. B. Blake, J. F. Fennell, R. S. Selesnick, T. G. Onsager, and H. E. Spence,
Title: Relativistic Electron Flux Variations: A New, Global, ISTP Perspective,
Reference: AGU Fall Meeting, San Francisco, CA, 8-12 December, 1997.
Reference Type: Invited Talk
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Abstract:
Coordinated observations from ISTP and other, programmatic missions have yielded a new understanding of relativistic electron events Ð magnetic storms which produce large enhancements in the fluxes of MeV electrons in the inner magnetosphere and which can severely impact the operation of spacecraft. Previous studies have suggested that relativistic electrons are either (a) accelerated slowly, possibly through a "recirculation" process, such that fluxes peak several days after the main phase of a storm or (b) accelerated in mere minutes though shock acceleration within the magnetosphere. However, neither of these processes, singularly or in combination, is capable of producing the spatial and temporal flux profiles that were observed during the January 1997 storm. In this paper we present comprehensive observations of relativistic electron fluxes during the January 1997 storm. We show that, contrary to all previous models, the fluxes of relativistic electrons were enhanced quickly, but continuously, over a period of 12 hours beginning at about 1200 UT on January 10. Fluxes were enhanced at all L-shells (3.5-6.6) essentially simultaneously. At geosynchronous orbit two flux maxima were observed. The first maximum was observed late on January 11. It appears to have been produced by a quasi-adiabatic, outward motion of electrons in response to compression of the magnetosphere. As the magnetosphere became less compressed the flux vs. L profiles were observed to relax back to lower L-shells. The second peak was observed several days after the passage of the interplanetary magnetic cloud. The intensity of this second peak diminished closer to the Earth until at L=4.6 it could not be observed. Radial profiles of relativistic electron fluxes suggest that the delayed peak which is commonly observed at L=6.6 is the result of outward radial diffusion from a more stablly trapped source near the peak of the outer electron belt. These observations provide, in essence, a new model of the flux variations observed in the inner magnetosphere during relativistic electron events. This model consists of (1) continuous acceleration of electrons throughout the inner magnetosphere on a time scale of approximately one half day, (2) quasi-adiabatic or fully-adiabatic transport of particles in response to compressions of the magnetosphere, and (3) outward radial diffusion (and loss) of particles from a relatively stabally trapped population inside of approximately L=5. The new ISTP observations also impose important new constraints on the mechanism which is responsible for the initial acceleration of these "killer electrons".
