Dependence of coherent nonlinear whistler interaction on wave amplitude

Abstract

Dependence of coherent nonlinear whistler interaction on wave amplitudes is investigated by both computer simulations and accompanied theoretical consideration. Six self‐consistent computer simulations have been carried out for six different intitial amplitudes of whistler waves. The computer code follows Newtonian dynamics of 3072 superparticles in a self‐consistent wave field. In four cases, a monochromatic whistler wave grows up exponentially and subsequently shows an amplitude oscillation around a saturation level. The saturation levels in these four cases are, however, almost the same regardless of the initial wave amplitudes. In the other two cases in which the initial wave amplitudes are larger than the saturation level of the previous four cases, the whistler wave shows neither growth nor damping but merely shows gradually decaying amplitude oscillations. The difference in nonlinear behaviors of resonant electrons in these six computer runs such as phase trapping, phase bunching, and pitch angle diffusion is also considered and discussed. A theoretical interpretation of the simulation results is given based on a rather simple model. An analytical expression for the saturation level is derived based on the model, which shows a good agreement with our simulation results.