Dynamics of ring current ions as obtained from IMAGE HENA and MENA ENA images

Abstract

Ion distributions from three storms in the year 2000 have been obtained from HENA and MENA ENA images at 27–39 keV and 5–12 keV, respectively. The observed motion of the ring current is compared with calculations of expected radial and angular drift speeds made using a combination of the Tsyganenko 89 magnetic field model and the Weimer 2001 electric field model. In one case between 1000 and 1100 UT on 10 June 2000 that had already been reported, the westward drift of the 33 keV ions is as expected. Comparisons with speeds inferred from the motion of peaks in the ion distributions, however, often did not agree with the model predictions. Examination of the predicted paths showed that the ions are expected to experience significant energy changes as they drift so that they quickly move out of the observed energy channel. Thus we are able to conclude that the observed motion is consistent with equatorial electric fields in the inner magnetosphere of a few tenths of a mV/m on 10 June 2000, several mV/m on 12 August 2000, and ≤1 mV/m on 4 October 2000 as predicted. Other interesting observations include (1) the suggestion of a two step injection process during the main phase of the 4 October 2000 storm, (2) the unexplained disappearance of the 33 keV protons when their drift motion brings them to noon in the recovery phase of the 10 June 2000 storm, (3) the apparent subtle changes in the electric potential pattern in the equator early in the 4 October 2000 storm, and (4) the energy dependence of the spatial source of the injected particles in the plasma sheet during the strong storm on 12 August 2000.