Electrostatic hydrogen‐cyclotron wave emission below the hydrogen‐cyclotron frequency in the auroral acceleration region

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Abstract
A mechanism is suggested for the excitation of electrostatic ion‐cyclotron waves at frequencies below the ion‐cyclotron frequency in the midst of the auroral acceleration region, which is assumed to consist of downward moving double layers. The mechanism involves upward flowing ions interacting with a downward flowing background plasma. The upward flowing ions are the ion beams accelerated by the double layer. The downward motion of the background plasma corresponds to a plasma expansion into the density cavity that develops in connection with double layer formation in the acceleration region. The ion‐cyclotron waves excited by the counterstreaming flows are doppler shifted to frequencies below the harmonics of the ion cyclotron frequency. It is suggested that such wave emissions could account for some very narrow‐banded waves at frequencies below the hydrogen cyclotron frequency that were observed by the S3‐3 satellite.