Electron resonances observed with topside sounders

Ionograms recorded with ionospheric sounders aboard rockets and satellites show signals (resonances) which can persist from a fraction of a millisecond to many milliseconds after the termination of the transmitted pulse. Many of the characteristics of the resonances at the plasma frequency fN, the upper-hybrid frequency fT, the harmonic gyrofrequencies nfB, where n ≥ 2, and the maximum frequencies of the Bernstein modes fQn can be explained by propagating electrostatic waves. At frequencies near fN, fT, and nfB, electrostatic waves of slightly different frequencies generated by the transmitted pulse propagate in the ionospheric plasma, become reflected at distances up to several hundred meters away from the satellite, and return to the satellite, producing a continuous receiver response following the transmitted pulse. The resonance observed at the gyrofrequency fB is not yet understood. Nonlinear properties of the receiving system and/or the plasma can result in resonances observed at the sum and difference frequencies of the principal resonances. Other resoncance phenomena are also discussed.