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Keywords:

  • Magnetosphere

Resonances associated with the maximum frequencies of cyclotron-harmonic waves (Bernstein modes) are observed on Alouette and Isis ionograms at both high and low latitudes; these are called the fQn resonances. Dispersion equations for oblique propagation in a hot magnetoplasma have been used to do ray tracing at frequencies near fQ2. The waves responsible for the observed resonance effects have group velocities that are very nearly equal to the the satellite velocity for the duration of the observed resonance; in contrast, other workers using ray tracing have found that the group velocity for the plasma-frequency and upper hybrid-frequency resonances changed considerably in a time equal to the observed resonance duration. The decay of the observed resonance with time is not due to Landau or collisional damping and hence may be due to spatial spreading. At low latitudes, the fQn resonances usually float, that is, the signals are weak immediately after the transmitter pulse. The resonance signals also often exhibit an interference pattern.