Auroral ion composition during large magnetic storms

Abstract

Ion-composition measurements from the suprathermal mass spectrometer (SMS) on AKEBONO (EXOS-D) during large magnetic storms (minimum Dst < −50 nT) reveal substantial changes in the mass composition of the auroral ionosphere. At quiet times, H+ and O+ ions dominate the high-altitude (> 1000 km) polar (Λ > 70°) ionosphere, and the minor-ion species (He+, N+, and O++) typically constitute about 10% of the total ion population: N+/O+ ≈ 0.05–0.1 and O++/O+ ≈ 0.1–0.2. During magnetic storms, the relative abundance of minor ions increase substantially, and N+ becomes a significant, at times dominant, component. During the main phase of large storms, the N+/O+ ratio reaches peak values of unity in the dayside. Likewise, the peak O++/O+ ratio reaches 0.4–0.5 at storm time. In addition, molecular [Formula: see text] and NO+ ions are occasionally present, and constitute ~5% of the total ion flux. The observed ions typically have energies of 5–20 eV/q, and are moving upward along the field line. They are believed to originate from the topside ionosphere. The increased abundance of N+ ions during large magnetic storms is believed to be a direct result of the increase in molecular nitrogen density in the F-region and topside ionosphere due to thermospheric heating in the presence of prolonged auroral activity. This implies that N+ ions may possibly be the dominant plasma component in the magnetosphere during very large magnetic storms.