Change of the plasma sheet ion composition during magnetic storm development observed by the Geotail spacecraft

Abstract

The present study aims to investigate how and where ions of ionospheric origin are accelerated to the ring current energy (a few tens to a few hundreds of keV) and how they are supplied to the ring current. We examined the plasma sheet ion composition during magnetic storm development, using energetic (9–210 keV) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. We selected two magnetic storms, that is, the 16–17 May 2000 storm and the 25 December 1998 storm, for which the energy density ratios of O+/H+ and He+/H+ in the plasma sheet were calculated from the EPIC/STICS data. These magnetic storms had a minimum of the SYM‐H index (the 1‐min Dst index) less than −50 nT and a duration of the main phase shorter than 6 hours. We obtained the following results: (1) Both the O+/H+ and He+/H+ energy density ratios were anticorrelated with the SYM‐H index (∣r∣ = 0.73–0.88); (2) The O+/H+ energy density ratio was rather constant at ∼0.1 before storms, but reached 0.3–1.0 at the storm maximum; and (3) The He+/H+ energy density ratio increased from 0.01–0.02 before storms to 0.04–0.1 at the storm maximum. These ion composition changes are comparable to those in the ring current, which have been reported by previous studies, indicating that ions of ionospheric origin are possibly convected to the ring current via the plasma sheet. A close inspection of ion energy spectra revealed that the observed ion composition changes can be attributed to the mass‐dependent acceleration of ions by the dawn‐to‐dusk electric field in the current sheet and the additional transport of ionospheric ions into the plasma sheet.