On the origin of sporadic keV ion injections observed by Interball‐Auroral during the expansion phase of a substorm

Abstract

During the expansion phase of a substorm on October 10, 1997, the Interball‐Auroral probe traveling across the auroral zone in the midnight sector detected sporadic injections of H+ and O+ ions with energies of a few keVs. We show that these injections are of ionospheric origin and are related to the large fluxes of low‐energy (a few tens of eV) upflowing ions near the poleward boundary of the auroral oval. Using test particle trajectory simulations, we demonstrate that these low‐energy ionospheric ions may be subjected to a rapid (on the timescale of expansion phase) circulation inside the plasma sheet. After their ejection from the poleward propagating auroral bulge, these ions travel into the midtail where they behave nonadiabatically and experience energization up to the keV range. As a result of magnetic moment damping in the current sheet, some of these ionospheric ions escape from the equatorial magnetosphere and travel back toward low altitudes. Numerical calculations reveal that upon reaching Interball‐Auroral near ∼3 RE altitude, these particles give rise to the observed keV injections, the sporadic character of which is due to the chaotic nature of ion motion in the magnetotail.