The onset phase of upstream energetic (>25 keV) ion events: Interball-1/DOK-2 observations

Abstract

The rise time of ions of various energies to reach the “plateau” phase of an event is a crucial parameter in the research of upstream ion events. In this paper, we present observational results on various parameters related with the onset phase of a large number (242) of upstream events. Our study was based on the statistical analysis of high time resolution energetic (> ∼ 25 keV) ion data obtained by the DOK-2 experiment onboard the Interball-1 spacecraft in the years 1996 and 1997. The main results of the study are the following: (1) The average rise times for ∼ 105 keV and ∼ 58 keV ions are <T 105 > ≊ 9 min and <T 58> ≊ 6.6 min , respectively, (2) The average delay between the beginning times of the plateau phase of events at ∼26 keV and ∼105 keV is <| 26−T 105| > ≊ 5.6 min , and (3) The percentage of events displaying forward, inverse and non energy dispersion between the beginning of the plateau phases of ion fluxes (at ∼105 keV and ∼26 keV) was evaluated to be ∼35%, 32% and 33%, respectively; i.e. 68% do not show inverse energy dispersion. These findings are consistent with the leakage model of magnetospheric particles, although appear to pose serious problems to most Fermi type acceleration models as applied at the Earth's bow shock.