Ionospheric plasma resonances: Time durations vs. latitude, altitude, and [formula omitted

Abstract

The variation of resonant time duration with latitude, altitude, and ⨍ N/⨍ H has been determined for the plasma resonances observed by Alouette I and Alouette II at the electron plasma frequency ⨍ N the electron cyclotron frequency ⨍ H , the upper hybrid frequency ⨍ T , and the harmonics n⨍ H where n = 2,3⋯. The long duration form of the ⨍ N , resonance (which is required in order to make electron temperature measurements based on the oblique echo theory) is only observed when the transmitted sounder power is greater than approximately 0.5 W. The ⨍ H resonance has a time duration longer than any of the n⨍ H resonances in a rare plasma (about 10 3 electrons/cm 3); in a dense plasma (about 10 5 electrons/cm 3), on the other hand, it is one of the weakest resonances observed. The 2⨍ H resonance is the only n⨍ H resonance that does not show a definite dependence of duration on latitude and the only one that shows a strong dependence on ⨍ N/⨍ H ; the other n⨍ H resonances show a definite dependence on altitude (as well as latitude). The resonances at 3⨍ H and 4⨍ H are observed strongest at high altitudes whereas the high order harmonics are consistently observed only at low altitudes. The n⨍ H observations with n > 5 are consistent with a stationary resonant region, extending only about 2 meters from the antenna, that is observed during its full time decay period (up to a few msec) in low latitudes; the observations are limited in high latitudes by the motion of the satellite. The results show that neither the theory based on longitudinal plasma waves with low group velocity excited by an infinitesimal dipole (which predicts much longer time durations) nor the theory based on magnetic field nonuniformity limitations of non-longitudinal plasma waves propagating with a group velocity matched to the satellite velocity, gives a proper interpretation of the high order nf H resonance observations.