Ion dynamics and tail current intensification prior to dipolarization: The June 1, 1985, event

Details Ion dynamics and tail current intensification prior to dipolarization: The June 1, 1985, event Ion dynamics and tail current intensification prior to dipolarization: The June 1, 1985, event

Nov 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000jgr...10525233o&link_type=abstract

Journal of Geophysical Research, Volume 105, Issue A11, p. 25233-25246

Physics

11

Magnetospheric Physics

Scientific paper

The present study investigates the tail current intensification prior to dipolarization, focusing on the dynamics of energetic ions. The event selected for this study, which was initially reported by Lui et al. [1988], occurred on June 1, 1985. The uniqueness of this event is that the AMPTE/CCE spacecraft remained close to the neutral sheet until the local magnetic field was dipolarized. Energetic (30-200 keV) ion fluxes were larger when the telescope was looking dawnward than when it was looking in the opposite direction. Until several tens of seconds before the (local) onset of dipolarization, this dawn-dusk flux anisotropy was more manifest for higher-energy ions, suggesting that the anisotropy was caused by the radial gradient of the energetic particle density. That is, the tail current was driven by radial pressure gradient. However, just prior to the onset, the flux level and the dawn-dusk anisotropy of the lowest-energy (31-43 keV) channel increased significantly, causing a further intensification of the tail current. The feature was less obvious in higher-energy channels. This energy dependence is better explained in terms of ion motion across the magnetic field. The velocity of this motion is a function of the ion energy and is estimated at 4% of the velocity corresponding to a given energy. It is likely that the associated enhancement of the tail current density was comparable to the enhancement during the entire growth phase. A positive feedback process between the unmagnetization of ions and the thinning of the current sheet is proposed for explaining this explosive intensification of the tail current. It is also found that electron pressure anisotropy did not contribute to the tail current intensification in the present event. The present result strongly suggests that the ion kinetics is important in the tail current intensification prior to the local onset of tail current disruption.

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