An experimental study of ELF/VLF hiss generation in the Earth's magnetosphere

Simultaneous measurements of whistler mode wave spectra in the frequency range 200 Hz to 3 kHz and of electrons in the energy range 15‐300 keV obtained on board the satellite GEOS 1 or GEOS 2, allow us to study ELF hiss generation. This study is performed both inside and outside the plasmasphere by computing the wave growth rate from the data at different magnetic latitudes λ m (0 ≤ λ m ≤ 30°). Inside the plasmasphere it is found that the integrated spatial gain Γ > 100 dB, is large enough to account for the observed wave intensity (∼1 pT² Hz −1 ) in a single transit of the wave through the equatorial region (λ m ≤ 20°), which appears as the preferential amplification region. Outside the plasmasphere the situation is more intricate. The total gain Γ ranges from 16 dB to 70 dB, and the observed wave intensities are often larger than inside the plasmasphere; they could be obtained by 2‐3 transits of the waves through the equatorial region with reflection at high latitudes. This does not exclude a contribution due to amplification of exohiss leaking out of the plasmasphere at high latitudes. Both inside and outside the plasmasphere, the patterns considered to explain hiss generation which imply small values of the angle θ between the wave normal and the static magnetic field (i.e. θ < 30°) within a limited latitude range around the magnetic equator (λ m ≲ 10°), are well supported by recent studies of the propagation characteristics of ELF hiss observed on board GEOS 1.