Propagation characteristics of dayside low‐altitude hiss: Case studies

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Abstract
A comprehensive set of wave analysis techniques is applied to dayside ELF hiss observed over three typical orbits of the low‐altitude AUREOL 3 satellite. Validity domains are established for each technique, whose results indicate the following propagation characteristics. Within the plasmasphere the waves are narrow‐band, have a lower cutoff frequency close to the local proton gyrofrequency ƒH+, and propagate downward with very oblique k vectors directed toward lower L values. Left‐hand mode waves are detected just below ƒH+. Within the plasmapause gradient the waves are broad‐band, have a lower cutoff frequency at the vicinity of ƒH+, and propagate mainly upward with oblique k vectors directed toward lower L values. Within the light‐ion trough region and the auroral zones the waves are broad‐band, have lower cutoff frequencies that can be below the local ƒH+, and propagate downward with k vectors along B0. Narrowband emissions detected in the vicinity of the cusp propagate upward. Exceptions are found at frequencies just above ƒH+ where, at nearly all invariant latitudes, waves are commonly upgoing. We conclude that (1) the reflection at the two‐ion cutoff frequency is a very efficient process for returning waves to higher altitudes and (2) the large density gradients that characterize the plasmapause and the cusp seem to act as traps for waves reflected below the satellite.