Physics
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusmsm22a..15h&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #SM22A-15
Physics
2700 Magnetospheric Physics, 2764 Plasma Sheet, 2788 Storms And Substorms
Scientific paper
A common feature of magnetospheric substorm is the injection of energetic particles with energies of tens to hundreds of keV observed in the inner magnetosphere. Since this injected population can readily be subjected to an energy-dependent azimuthal drift around the Earth, it appears to be a flux enhancement with energy dispersion away from the injection region, which has been referred to as a drift echo. We found that such a drifting population with a fairly long (~several tens of minutes) dispersion can be observed at a distance of > 9 Re, indicating that energetic particles can make a closed drift path around the Earth even much further away than at the geosynchronous distance. In the present paper, we investigate spatial and temporal characteristics of such drifting populations in detail by focusing on an interesting event observed by the GEOTAIL spacecraft. In this event, GEOTAIL was situated in the plasma sheet around the post-midnight sector at a distance of ~ 10 Re and observed a drift echo of energetic protons with energies of several tens to hundreds of keV. This drifting population showed a clear azimuthal sector anisotropy in particle distribution as it passes by the spacecraft: There is lack of duskward-flowing particle fluxes at the leading edge of the drift echo, and subsequently the distribution becomes isotropic in the middle of the event, and finally enhanced duskward-flowing fluxes are observed around its trailing edge. Since the magnetic field is dominated by the Bz component during this event, this sequence of anisotropy can be considered in terms of a radially confined structure of the drifting population of a scale of the order of proton gyrodadius (~0.5 Re, in this case). Because the duskward-flowing flux at the spacecraft corresponds to a proton with its guiding center sunward of the observation point, it is suggested that the drifting proton echo has a spatial structure such that protons are localized tailward of the spacecraft at the leading edge, while a sunward density gradient exists around the trailing edge. It will be shown that this result can be interpreted as a spiral-shaped leading edge of drifting population which naturally results from the radial dependence of the particle fs longitudinal drift speed. We have made a simple drift path calculation based on the guiding center approximation and have found that such a spiral-shaped structure can be formed at the forefront of the drifting protons.
Hori Toshihiro
Lui A. T.
Maezawa Kiyoshi
McEntire R.
Mukai Tadashi
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