Side band and harmonic radiation from topside sounders

The power produced by the Isis 1 and Isis 2 topside sounders at frequencies other than the transmitter frequency can be deduced from ionograms produced by mixed mode sounding. In this mode the transmitter remains at a fixed frequency while the receiver performs its normal sweep in frequency. On occasion, ionospheric echoes are observed at frequencies other than the nominal transmitter frequency. The echoes result from power produced at the side bands and harmonics of the transmitter frequency. Ionospheric echoes resulting from side band radiation are observed only when a spread condition at the satellite is observed on the ionograms, indicating that the propagation is guided (ducted) along the field-aligned irregularities present. The harmonic radiation observed could be due to small distortions of the signal in the transmitter, distortions introduced by ion sheath, or by nonlinear processes in the plasma surrounding the antenna. The power generated at the side bands and harmonics of the nominal transmitter frequency may be relevant in any theories that are developed to explain particular resonances.