During a close rendezvous of the ISIS I and ISIS II topside sounder spacecraft, medium-frequency signals transmitted from the latter to the former exhibited two kinds of anomalous enhancement, one at about twice the electron gyrofrequency 2fc and another at a separation below the RF carrier approximately equal to fc. Signal enhancements of up to 50 dB above those calculated with classical theories of transmitting and receiving antennas were observed. The sounder data clearly show that these signals propagated in the normal cold plasma upper branch O and X modes. Natural radio emissions in the form of whistler mode and Z mode hiss were recorded at the same time, implying the presence of energetic particles. Second-harmonic cyclotron amplification along the ray path connecting the transmitter and receiver has been evaluated and found unsatisfactory for explaining the 2fc enhancement. The postulate that a plane wave decay nonlinearity is enhanced by soft electron fluxes that also produce the observed hiss has also been considered and rejected. Nonlinear electron motion in the near field of the transmitting antenna might produce both types of enhancement.
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