CEP as a Source of Upstream Energetic Ions

Abstract

The cusp energetic particles (CEP) have been observed in the dayside high-altitude cusp region, showing orders of the magnitude increase of ion intensities with energies from 20 keV up to 10 MeV. Associated with these charged particles are large diamagnetic cavities with significant fluctuations of the local magnetic field strength. The CEPs may provide a answer to a long-standing unsolved fundamental issue about the origin of upstream energetic ions. On June 28, 1999, the WIND spacecraft (near the forward libration point) observed a sudden increase (by more than one order of magnitude) of the solar wind pressure at about 4:45 UT and an upstream ion event at 5:23–5:45 UT, the INTERBALL-1 spacecraft located just upstream of the bow shock in the morningside measured an upstream ion event from 5:16 UT to 6:00 UT, the GEOTAIL spacecraft in the afternoonside near the bow shock detected an upstream ion event from 5:50 UT to 6:16 UT, while the POLAR satellite at 7 hours of magnetic local time detected an energetic particle event in the high-altitude region associated with turbulent diamagnetic cavities from 5:12 UT to 6:30 UT. Energetic oxygen ions of both ionospheric and solar wind origin were observed by the POLAR spacecraft during this event period. The energetic ions and the associated turbulent magnetic field are very similar to what was found in the high-altitude dayside cusp region. It is argued that the bow shock is not the main source of energetic ions in these upsteam events since their energy spectra are independent of the solar wind velocity and their intensities are independent of the bow shock geometry and solar wind pressure. The event onset was first detected in the cusp by POLAR at 5:12 UT, then near the bow shock in the morningside by INTERBALL-1, and then in far the upstream by WIND. The measured energetic ion intensity decreased with increasing distance from the cusp before 5:42 UT. At 5:50 UT, GEOTAIL detected the event onset that showed an energy dispersion, suggesting a drift effect. These observational facts together with the IMF directions suggest that these upstream energetic ions most likely came from the cusp.Key wordscuspenergetic particlesbow shockupstream events