The plasma wave environment of an auroral arc: Electrostatic ion cyclotron waves in the diffuse aurora

Abstract

Emissions that appear to have been electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. The rocket was launched from Poker Flat, Alaska, at ∼2030 MLT. The flight successively traversed ∼70 km of the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. In the diffuse aurora, peaks were observed in the power spectrum of the electric field at frequencies near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave spectrum parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H+ and O+ EIC waves. Taken together, the two analyses indicated that both emission bands were due to waves propagating both up and down the field line and eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting large Landau damping require that the source be local. Free energy for the waves was apparently available in the 5 µA/m² downward parallel current density which was inferred from the magnetometer data. The presence of the waves indicates that this current was being carried by less than 2% of the plasma, presumably in the form of a field aligned beam of electrons with energies of a few eV.