Substorm ion injections: Geosynchronous observations and test particle orbits in three-dimensional dynamic MHD fields

J. Birn, M. F. Thomsen, J. E. Borovsky, G. D. Reeves, D. J. McComas, R. D. Belian, M. Hesse

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We investigate particle acceleration and the flux increases associated with substorm particle injections using geosynchronous observations and test proton orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. The energetic particle flux changes obtained from the test particle orbits agree well with observations that demonstrate rapid ion flux increases at energies above ∼20 keV but little change at lower energies. The "injection region" inferred from the test particles not only has a sharp earthward boundary (the usual injection boundary) but also a sharp but ragged tailward boundary. The earthward portion of enhanced ion flux can be traced to the enhanced cross-tail electric field associated with the collapse and dipolarization of the inner tail, whereas the tailward edge is closely associated with the near-Earth x-type neutral line. Because of the rapid earthward motion of accelerated ions away from the neutral line, this boundary is displaced earthward to where the energetic ions become more adiabatic in the stronger dipolar field. Lower-energy ions are not affected by cross-tail acceleration in the strong Ey fields because their earthward E×B drift dominates the cross-tail drift, except very close to the neutral line.

Original languageEnglish (US)
Article number96JA03032
Pages (from-to)2325-2341
Number of pages17
JournalJournal of Geophysical Research: Space Physics
Issue numberA2
StatePublished - 1997
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Oceanography
  • Forestry
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Space and Planetary Science
  • Earth and Planetary Sciences (miscellaneous)
  • Palaeontology


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