Observation of electromagnetic oxygen cyclotron waves in a flickering aurora

Abstract

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field‐aligned Poynting fluxes of up to ∼10 −5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground‐based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.