Low altitude acceleration of ionospheric ions

Abstract

Energetic ion and electron observations were made from a high altitude sounding rocket launched into the expansive phase of an auroral substorm near local midnight. In the 400‐500 km altitude range, on both the up‐ and down‐leg, intense (∼10 8 cm −2 sec −1 sr −1 keV −1 ) beams of 90‐500 eV ions were observed at 90° pitch angles. At higher altitudes the beam appeared at larger pitch angles, the variation of these pitch angles with height being consistent with adiabatic expansion of a population injected at 90° at lower altitudes. The width of the beam was within the angular response (∼10°) of the ion sensor at all energies and altitudes. These ions, energized up to 500 eV, were observed throughout the flight, enhancements occurring at the edges of auroral precipitation regions and, for the largest event, in coincidence with intense field‐aligned electron precipitation. Acceleration by ion cyclotron waves produced via a current‐driven or beam‐plasma instability is suggested as a possible source mechanism.