VLF emission triggering by a highly anisotropic energetic electron plasma

A recent paper (Bell et al., 2000) reports observations from the POLAR spacecraft of highly anisotropic hot electron distribution functions in the equatorial region of the magnetosphere at L = 3.4. The particle instrument HYDRA measures electron fluxes from 1–20 keV. VLF emissions triggered by pulses from Omega (Norway) are found to coincide with “pancake” type electron distributions with average pitch angles >70 degrees, such distributions being effectively confined to the equatorial zone. We examine the linear and nonlinear wave particle interaction process between pancake distributions and continuous wave (CW) or narrow band ducted whistler mode signals. It is concluded that the pitch angle range of 67–76 degrees dominates the interaction process, and that with in-duct wave saturation amplitudes of 6 pT strong nonlinear trapping occurs for these particles. Using these data, a 1-D Vlasov Hybrid Simulation VLF code was run to simulate numerically risers triggered by a 1 s Omega pulse. The integrated linear trans-equatorial amplification of ∼15 dB agrees well with figures calculated by Bell et al. (2000) from the HYDRA data. Fallers, hooks and oscillating tones have also been simulated.