Influence of heavy ionospheric ions on substorm onset

Multifluid/multiscale simulations are used to examine the influence of ionospheric outflows on two substorms that occurred on August 13, 2001. Both substorms had well defined onsets in Cluster ion spectrometer (CIS) data of ion composition of the plasma sheet. It is shown that the model is able to account for two orders of magnitude variation in the H+ density and one order of magnitude change in the O+ density in the plasma sheet in association with the ionosphere generating peak outflows of 4 × 1025 H+ ions/s and 2 × 1025 O+ ions/s. The model shows that the growth phase is associated with the venting of solar wind plasma in association with the formation of an X‐line and the ejection of a plasmoid. This reconnection occurs in a proton‐dominated plasma and occurs well before onset. After this venting, a Y‐line configuration develops with the ionospheric plasma being the dominant source of the plasma sheet. Lobe reconnection involves heavy enriched O+ lobe field lines produced by enhanced outflows that start at the beginning of the growth phase. This O+ produces enhanced dissipation and localized formation of flux ropes prior to substorm onset. For the isolated substorms considered here, the acceleration of O+ in the plasma sheet keeps the density low in this region but leads to the buildup of both number and density at the inner edge of the plasma sheet. O+ can contribute nearly 50% of the total energy density in this region just prior to onset. This results in over pressurization and dipolarization in conjunction with onset and associated increases in the nightside auroral currents. Key Points Heavy ionospheric ions influence onset Reconnection occurs much earlier than onset Lobe reconnection and the inner edge of the plasma sheet are contributing factor