Dynamics of ionospheric O+ ions in the magnetosphere during the 24-25 September 1998 magnetic storm

We have used the time‐dependent electric and magnetic fields obtained from a three‐dimensional global magnetohydrodynamic (MHD) simulation of the magnetosphere and large‐scale kinetic particle tracing to investigate the population of the magnetosphere by O+ ions from the dayside ion fountain during the 24–25 September 1998 magnetic storm. The MHD simulation used WIND data upstream of Earth as input. Ions were launched from both hemispheres on the dayside, and the ion launch rate was dynamically normalized to observations. We found that ionospheric O+ ions had access to the plasma sheet beyond a radial distance of 10 RE before the storm but gained access to the near‐Earth region and partial ring current soon after the storm sudden commencement (SSC). O+ ions already en route to the plasma sheet were accelerated and injected into the region nearly instantaneously after the SSC, resulting in a significant increase in the plasma sheet O+ number and energy densities. Significant changes to the magnetospheric configuration caused by the variations in solar wind dynamic pressure and IMF By and Bz, most notably the two pressure peaks at 000 UT and ∼0140 UT and the southward turning of the IMF at 0140 UT resulted in a relative absence of O+ ions from the magnetotail extending from ∼0140 UT to ∼0300 UT. After 0300 UT, the O+ density in the plasma sheet once again increased to >1 cm−3, and O+ was more abundant in the magnetotail compared even to the period immediately following the storm commencement. Prior to the storm, the average O+ energy density in the plasma sheet and ring current was 0.1–3 keV cm−3 and increased 20–30 keV cm−3 during the main phase of the storm, with peak values reaching 100 keV cm−3, consistent with the observations reported by Nosé et al. (2005).