Simulation Study of the Direct Measurement of the Pitch Angle Scattering of Energetic Electrons by Whistler-Mode Chorus Emissions

Abstract:

Wave-Particle Interaction Analyzer (WPIA), which is a new instrumentation proposed by Fukuhara et al. (2009), measures a relative phase angle between a wave magnetic field vector and a velocity vector of each particle and calculates the energy exchange from waves to particles. The WPIA has a potential to directly detect wave-particle interactions in space plasmas and will be installed on the ERG satellite of JAXA/ISAS. In the present study, in addition to the energy exchange proposed by Fukuhara et al. (2009), we propose the direct measurement of the pitch angle scattering of resonant particles by plasma waves via the WPIA by computing the Lorentz force of wave electromagnetic fields acting on each particle. We apply the proposed method to results of the one-dimensional electron hybrid simulation reproducing the generation of whistler-mode chorus emissions around the magnetic equator [Katoh and Omura, 2007]. By using the wave and particle data obtained at fixed observation points assumed in the simulation system, we analyze the Lorentz force acting on each particle and compute the averaged force in the whole simulation time, corresponding to 20,000 gyro periods. We use 200 keV electrons and the time, kinetic energy, and pitch angle resolutions of 500 gyro-periods, ±10%, and 1 degree, respectively, for the analysis of the averaged Lorentz force. In the result of the analysis, we obtain significant values for electrons in the kinetic energy and pitch angle ranges satisfying the cyclotron resonance condition with the reproduced chorus emissions. The obtained value is three times larger than the magnitude of perturbations in other pitch angle ranges. We compared the result of the analysis with the temporal variation of pitch angle distributions and wave spectra observed at fixed points in the simulation. While the pitch angle distribution varies similarly in both hemispheres, the obtained Lorentz force is only significant in the pitch angle range corresponding to the electrons moving northward (southward) in the southern (northern) hemisphere, indicating the pitch angle scattering of electrons by pole-ward propagating chorus emissions. The results of the present study demonstrate that the proposed method enables us to identify the location where the pitch angle scattering occurs in the simulation system.