The Experimental Results of triggering Chorus Emissions from Monochromatic Wave Components in the Hiss Band in the Outer Magnetosphere

Abstract : The results of detailed spectral analyses and direction finding for both VLF and ELF data containing simultaneous hiss and chorus are reported on the basis of the data observed onboard the GEOS1 satellite. The following important results are obtained; (1)Each chorus element has a tendency to be originated from the hiss band and is asymptotic to the hiss band. (2)The intensity and occurrence of chorus are closely correlated with the intensity of the underlying hiss. (3)The hiss has been so far considered to be random and incoherent, but it exhibits some structures or wavelets, which are monochromatic wave components with significant duration, and we can notice the exsistence of the causative wavelet at the foot of each chorus element. (4)When we expect triggering a chorus from a wavelet at the upper edge of the hiss band, the duration of the wavelet increases with decreasing its intensity. (5)Very similar φ values of the azimuth are noticed for both hiss and chorus, suggesting that both phenomena come from the same source region. These experimental facts might lead us to conclude that a wavelet existing at the upper edge of the hiss band is able to generate a chorus emission through the coherent wave-particle interaction in the outer magnetosphere as in the case of active VLF wave injection experiments.