Consideration on dynamic spectra and direction finding results of hiss-triggered chorus emissions

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Based on our previous experimental results of hiss-triggered chorus emissions and the theory of Helliwell, we propose a possible mechanism for how a chorus emission is generated from a wavelet in the hiss band. It consists of two steps : the phase bunching process of the resonant electrons and the radiation process. We select a VLF event observed onboard satellite GEOS1 on July 21,1977 and verify the proposed mechanism computationally. Whether the phase bunching occurs or not is examined to investigate the phase bunching time, the resonance time, and the duration of the wavelet. The validity of the generation model is considered through the reconstruction of chorus elements with use of three-dimensional ray tracing computations. Special attention is paid to the slope of the chorus elements and the frequency dependence of the wave normal angles for a chorus element. The theoretically reconstructed chorus element which satisfies the temporal variations onboard the satellite is found to occur with the Gendrin angle for the generation angle at the equator. Furthermore, there is a tendency for the generation angle to the Earth's magnetic field to shift to a larger angle with increase of frequency and to terminate around the oblique resonance angle. Propagation after the radiation is also found to be in the non-ducted mode through the computations.
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