Computer simulations of precipitating energetic electrons through chorus-wave particle interactions

Abstract:

Whistler mode chorus waves cause scattering and acceleration of energetic electrons in the inner magnetosphere, and recent studies identified that chorus waves cause the pulsating aurora. The interaction processes have been modeled as diffusions in the velocity space, and it has been supposed that the scattering rate increases with increasing the wave amplitude. However, the wave-particle interactions with chorus waves are non-linear process, so that it is expected that the scattering rate does not simply depend on the wave amplitude. In this study, we investigate chorus wave amplitude dependence of electron scattering using the GEMSIS-RBW simulation code. The GEMSIS-RBW simulation calculates variations of local pitch angle and energy of each test particle by the imposed chorus waves. In this simulation, chorus bursts that consist of multi rising tone elements are imposed at the equatorial plane, and these waves propagate along the field line with L= 4 by calculating the Maxwell equations. We calculate the trajectory of a number of electrons with initial energy of 50 keV and evaluated the number of precipitating electrons with various wave amplitudes. In order to evaluate the number of precipitating electrons that depends on the wave amplitudes, we discriminate between resonant and non-resonant electron population. The number of the precipitating electrons through resonance simply increases when the wave amplitude increases from 10 pT to ~200 pT. However, as the wave amplitude increases more than 200 pT, the number of precipitating electrons decreases. From the analysis on the electron motion in the phase space as well as the parameter ρ [Bortnick et al., 2008] that is a proxy of the ratio of the wave-induced and the background inhomogeneity effects for the momentum change of the resonant electron, the phase trapping effect decreases the precipitating flux in the large amplitudes. On the other hand, the number of precipitating electrons through non-resonance increase, indicating that non-resonant interactions contribute to the total number of precipitating electrons.