INTERPRETATION OF SATELLITE GYROHARMONIC RESONANCE OBSERVATIONS

An analytical expression is derived that describes the rendezvous conditions between a propagating wave near nƒH and a moving ionospheric sounder antenna, where ƒH is the ambient value of the electron gyrofrequency and n is an integer greater than 1. The agreement between the theoretical predictions and Alouette 1 satellite observations indicates that most of the long duration resonances (those with time durations greater than about 2 msec) can be interpreted in terms of the reception of sounder‐stimulated electrostatic waves that are reflected and returned to the sounder antenna. The reflection is due to the sensitivity of the dispersion relations near nƒH to small changes in the ambient magnetic field . The n = 2 resonance is the exception because the satellite observations cannot be explained by the analytical expression obtained for the oblique echo theory. The results provide a third technique for obtaining T⊥ information from sounder‐stimulated plasma resonances, where T⊥ is the ambient electron temperature corresponding to electron motions perpendicular to . The relative variations in T⊥ can be observed to an accuracy of a few percent by recording the relative variations in the nfH resonant time duration if the angles defining the antenna geometry are known to an accuracy of a few degrees. Copyright 1979 by the American Geophysical Union.