Further evidence of triggering chorus emissions from wavelets in the hiss band

Abstract : The results of detailed spectral analyses and direction finding for ELF data containing simultaneous hiss and chorus have been reported on the basis of the data observed on-board GEOS 1 satellite in the outer magnetosphere at a high geomagnetic latitude (Λ ∼- 25°). A combination of the present results with those from our previous equatorial studies has yielded the following findings : (1) each chorus element has a tendency to originate from the hiss band and is asymptotic to the hiss band ; (2) the intensity and occurrence of the chorus are closely correlated with the intensity of the underlying hiss ; (3) the hiss band exhibits structures or wavelets (i.e. monochromatic wave component with significant duration) and we notice the existence of the causative wavelet at the foot of each chorus element ; (4) when we expect triggering chorus from a wavelet near the upper edge of the hiss band, the duration of the wavelet increases with decreasing intensity ; (5) very similar φ (azimuthai angle) values are noticed for both the hiss and chorus, suggesting that both phenomena come from the same source region. These observational facts might lead us to conclude that a wavelet existing near the upper edge of the hiss band is able to generate a chorus emission through coherent wave-particle interaction in the outer magnetosphere, as in the case of active VLF wave injection experiments.