Evidence for storm-time ionospheric ion precipitation in the cusp with magnetosheath energy

Abstract. We present evidence for a sporadic precipitation into the north polar cusp of ionospheric O⁺ and He⁺ ions accelerated up to the magnetosheath flow speed during a magnetic storm. This is deduced from data obtained on board the Interball-Auroral satellite showing that the energy/charge ratios of the H⁺, He⁺⁺, He⁺ and O⁺ populations are similar to those of ion masses. These measurements pertain to a very disturbed magnetic period. A storm was in progress with a D_st reaching -149nT during the cusp measurements, while the AE index reached values higher than 1000nT. This result is discussed in terms of ion circulation from the magnetosphere to the magnetosheath and back to the magnetosphere. We suggest that the acceleration of O⁺ and He⁺ ions up to a magnetosheath-like velocity is directly linked to the large B_y component of the IMF.

Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosheath; storms and substorms)