Low-energy ion precipitation during the Halloween storm

During the Halloween storm of October 29–31, 2003, four defense meteorological satellite program (DMSP) satellites detected fluxes of low-energy ions precipitating well equatorward of auroral electrons in the dawn/morning magnetic local time sector during the main phase. There were three southward turnings of the interplanetary magnetic field (IMF), leading to three intensifications of the ring current to DST values of −125, −310 and −350 nT. In each case the ion fluxes weakened dramatically and/or vanished during DST recoveries. DMSP only encountered short-lived episodes of subauroral ion precipitation in the dusk/evening sector. A brief survey of the DMSP database reveals that near-dawn, ion precipitation is a main-phase characteristic of all large magnetic storms. DMSP satellites also detected similar ion precipitation during the main phase of the March 1991 magnetic storm. During an outbound pass of the combined release radiation effect satellite (CRRES) through the inner magnetosphere it detected low energy (<1 keV) ion fluxes that were collocated with, but spectrally separated in energy from the ring current “nose structure”. As CRRES moved toward perigee in the dawn sector, it crossed similar low-energy (⩽500 eV) ions embedded in a broad ion population, located earthward of plasma sheet electrons. Simultaneously the DMSP F8 satellite detected spectrally similar low-energy ions precipitating at subauroral latitudes near dawn at the same invariant latitudes. To reconcile DMSP/CRRES observations with elementary concepts of allowed ion drift paths, the data suggest two source populations. The lowest-energy ions, of ionospheric origin, were initially energized earthward of the plasma sheet electron boundary in the evening local-time sector then co-rotated eastward. Higher-energy ions originated in the plasma sheet and drifted close to the Earth under the combined influences of time-varying convective electric fields and azimuthal gradients in the Earth's magnetic field generated by the stormtime ring current [Tsyganenko, N.A., Singer, H. J., Kasper, J. C., 2003. Storm-time distortion of the inner magnetosphere: how severe can it get? Journal of Geophysical Research, 108 (A5), 1209].