In a recent rocket experiment designed to resolve the slow-wave echoes comprising the resonances excited by topside sounders the plasma, the upper hybrid resonance, and the first three electron cyclotron resonances were observed. In particular, the variation of echo frequency versus delay was observed for the upper hybrid resonance. The good agreement ' found between the observed and the predicted frequency delay function confirms the slowwave propagation nature of the phenomenon. The frequency of all three electron cyclotron resonances are consistent with the Pogo (3/68) magnetic-field model within its stated accuracy.
Observations of resonances in the ionosphericplasma have been made by topside sounders since 1961 [Lockwood, 1963; Calvert and Goe, 1963; Calvert and McA•ee, 1969]. The experiments consist of exciting the plasma in the vicinity of the spacecraft with a short RF pulse and observing the response with a radio receiver. A resonance consists of an apparent ringing signal lasting a few milliseconds when the system is tuned to certain frequencies. The principal resonances occur at the plasma frequency f•, the upperhybrid frequency f•, and the electron cyclotron frequency fn and its harmonics nfn. Current explanations of the resonances involve the propagation, reflection, and reception of slow waves [McA]ee, 1968, 1969a, b, 1970a, b; Bitoun et al.A continuum of such slow-wave echoes can be found with a specific frequency delay relationship. This could produce a trace on a topside ionogram analogous to the normal electromagnetic-wave traces. However, the range of frequencies contributing to the resonance during a normal listening period is considerably narrower than either the rec.eiver bandwidth or the spectrum of the transmitter pulse. Therefore the echoes from a single pulse cover all delays to produce the apparent ringing signal. Previous satellite experiments were incapable of observing this frequency variation with delay within the receiver bandwidth because of on board signal detection.Previous evidence favoring the echo .explanation has relied on the prediction that two separate sets of echoes can occur when the appropriate conditions are met. One corresponds to waves launched with propagation vector components generally parallel to the spacecraft velocity, and the other to waves in the opposite direction. These two echoes may have different frequency delay variations, and, furthermore, they would experience different Doppler shifts. The difference in frequency between the two is about a kilohertz, so that an observable beat is produced in a sounder system by using amplitude detection. Quantitative comparisons between predicted and observed beat patterns yield convincing agreement [Warnock et al., 1970; Feldstein and Gra#, 1972]. This paper describes a rocket-borne sounder experiment. designed specifically to observe the slow-wave echoes comprising a resonance and reports the major results.
EXPERIMENTThe rocket sounder consisted of a frequency synthesizer, a pulsed transmitter, a receiver, a transm...