ISIS-I and ISIS-II observation of emissions triggered by doppler-shifted Norway Omega signals

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A great number of VLF spectra were collected by receiving the telemetry of ISIS satellites at Syowa Station, Antarctica, for about 6 years from May 1976 to February 1982. Norway Omega signals propagated in the non-ducted mode to the southern hemisphere and observed by the polar orbiting ISIS-I and ISIS-II satellites have shown very interesting latitudinal characteristics of their positive and negative doppler shift. According to EDGAR (J. Geophys. Res., 81,3327,1976), these characteristics are explicable by taking into account the wave normal direction at the satellite location, which can be estimated from the results of ray tracing. One very important result of our observation is that VLF emissions triggered by positively and negatively doppler-shifted Omega signals were discovered. From the doppler frequency shift of the triggering signal, we can determine their wave normal direction at the satellite as mentioned above, so that we have further tried to estimate their wave normal direction in the interaction region near the equatorial plane, where the triggered emissions are thought to have been generated. One such estimation for the case observed on November 28,1979 results in the wave normal angles at the equatorial plane to be around 10°-30°. Furthermore, in other Omega data, it can be shown that the wave normal angles for large doppler-shifted signals approach about 40°-60°at L<2.7.
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