VLF emission triggering by a highly anisotropic electron plasma

Abstract

A recent paper by Bell et al (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 non linear wave particle interaction process between pancake distributions and CW ducted VLF signals. It is concluded that the pitch angle range 67-76 degrees dominates the interaction process, and that with in duct wave saturation amplitudes of 6pT strong non linear trapping occurs for these particles. It is difficult to avoid the impression that highly anisotropic pitch angle distributions don’t have a great effect on resonant particle dynamics. High anisotropy has raised the pitch angle of maximum non linear contribution from 61->72 degrees, and reduced particle non linearity somewhat, in that the onset of trapping occurs at 2pT rather than 1.6pT. Using this data a 1D Vlasov Hybrid Simulation (VHS) VLF code was run to numerically simulate risers triggered by a 1 s Omega pulse. The VHS algorithm defines a time varying phase space simulation box covering the trans-equatorial nonlinear trapping region and a segment of parallel velocity space centred on the local resonance velocity. The simulation particles have F defined as a constant on their trajectories by Liouville's theorem. At each time step F is interpolated from the particles onto the fixed phase space grid, allowing resonant particle current to be calculated. The VHS method is extremely efficient since at each step particles leaving the phase box are discarded, and fresh particles are embedded into the phase fluid where the latter flows into the phase box. Successful numerical triggering of emissions by Omega is shown, and examples of risers, fallers and hooks are shown. The integrated linear trans-equatorial amplification of ~10dB agreed well with figures calculated by Bell from HYDRA data. These successful simulations of Omega emissions with highly anisotropic distribution functions confirm that non linear trapping of cyclotron resonant electrons in the geomagnetic field is the root plasma physical mechanism behind the triggering of VLF emissions.

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