Narrowband structure in HF waves above the electron plasma frequency in the auroral ionosphere

Abstract

We report high resolution measurements of the frequency‐time structure of HF waves just above the electron plasma frequency in overdense conditions in the auroral ionosphere. Short duration narrowband emissions (HF chirps) are observed whose frequencies predominantly decrease with time. Typical HF Chirp amplitudes are 1 mV/m, typical durations are 100 ms, and typical frequency‐time slopes are 50 kHz/s. The emissions often occur in flocks in which pairs of emissions appear simultaneously at frequencies 300‐2000 Hz apart and drift in parallel. The observed characteristics are consistent with propagating electromagnetic Z‐mode waves.