The role of Dayside O+ ions in the magnetosphere during the September 24-25, 1998 Magnetic Storm

We have used a global time-dependent magnetohydrodynamic (MHD) simulation of the magnetosphere and particle tracing calculations to determine the access of dayside ionospheric O⁺ ions to the storm-time near-Earth plasma sheet and ring current during the September 24-25, 1998 magnetic storm. The September 24 - 25, 1998 magnetic storm began with the arrival of a sudden impulse at 23:45 UT on September 24th which abruptly increased the solar wind dynamic pressure on the nose of the magnetosphere from 2 nPa to 15 nPa. Dst reached a minimum of -207 at ~1000 UT on September 25th. During this event, the TIDE instrument on board the POLAR spacecraft observed large-scale dayside ionospheric outflows in direct response to the impact of the pressure pulse on the magnetosphere. The MHD simulation was initiated with solar wind data obtained from the WIND spacecraft for this event. We traced a large number of particle orbits from the dayside ion fountain throughout the main and recovery phases of the magnetic storm. Particle launches were carried out at five minute intervals, and the ion outflow rates were normalized with empirical models as well as in-situ observations from POLAR/TIDE. This allowed us to assess the time-dependent access of ionospheric O⁺ ions to the magnetosphere during the storm, their energization in the magnetotail current sheet, their contribution, both in mass and energy density, to the inner magnetosphere, and their subsequent loss from the magnetosphere. Doing so also allows us to calculate the time-dependent contribution of O⁺ ions to Dst.