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

We report high resolution measurements of the frequency-time structure of H F waves just above the electron plasma frequency in overdense conditions in the auroral ionosphere. Short duration narrowband emissions (HF chirps) ar e observed whose frequencies predominantly decrease with time. Typical KF C hirp amplitudes are 1 mV/m, typical durations are 100 ms, and typical frequ ency-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 pro pagating electromagnetic Z-mode waves.