Harmonic H⁺ gyrofrequency structures in auroral hiss observed by high‐altitude auroral sounding rockets

Two recent sounding rocket experiments have yielded VLF wave data with spectral structures ordered by the hydrogen gyrofrequency. The spectral structures occur near and above the lower hybrid frequency in association with auroral hiss. These structures are observed within and near regions of auroral electron precipitation and transverse ion acceleration. They are accompanied by auroral hiss but are anticorrelated with spectral peaks at the lower hybrid frequency. They are typically found above 500 km altitude, have no measurable magnetic component, and are at least occasionally short wavelength ( k ρ i ≥ 1). Because the spectral structures appear to be electrostatic, are ordered by the hydrogen gyrofrequency, and are short wavelength, we interpret the structures as modes which connect the lower hybrid mode with the hydrogen Bemstein modes. A study of the plasma wave mode structure in the vicinity of the lower hybrid frequency is presented to substantiate this interpretation. Our results imply that these waves are a common feature of the auroral zone ionosphere above 500 km altitude and exist any time that auroral hiss exists. The absence of previous satellite observations of this phenomenon can be explained by Doppler broadening.