Rocket observations of two distinct types of dispersive features of auroral HF waves

Two dispersive auroral HF emissions have been observed on two sounding rockets 5 years apart, employing different sensors with different effective antenna lengths and payload orientations. Both the SIERRA and CHARM rockets were launched to ∼735 km over active auroral substorms from Poker Flat, Alaska. On both flights, two distinct types of dispersed features occurred, each of which exhibited a frequency-time structure characterized by an impulsive signal at one end of the frequency range, with progressively greater dispersion as the emission frequency approached a bounding frequency from either above or below. The first type of emissions, called "swishers," occurred in the frequency range 1200-1500 kHz, with a characteristic signature whereby the signal is progressively more delayed as its frequency approaches a lower bound from above. The second type of emissions, called "hooks," were observed in the frequency range 600-1100 kHz and were progressively more delayed as their frequency approached an upper bound from below. Qualitatively, hooks and swishers might both be explained purely by wave propagation: As the wave frequency approaches the lower cutoff of the Langmuir, O-mode, or Z-mode (in the case of swishers) or the upper cutoff of the whistler mode (in the case of hooks the group velocity becomes small and the signals are delayed. In the case of hooks, ray-tracing calculations support this explanation by showing that wave dispersion can explain the observed signature, if the emission originates on the whistler mode resonance cone on the 60° invariant field line. In the case of swishers, ray-tracing calculations suggest that wave dispersion alone cannot explain the observed signature for any possible mode.