Coherent whistler waves and oscilliton formation: Kinetic simulations

Abstract

Coherent wave emission in the whistler frequency range (0.1-0.5Ωe, where Ωe is the electron cyclotron frequency), consisting of nearly monochromatic wave packets, has recently been observed by several spacecraft in the Earth's plasma environment. In this paper whistler waves generated by an electron temperature anisotropy is studied using particle-in-cell (PIC) simulations. It is shown that during the initial phase of the evolution the exponential wave growth is consistent with linear kinetic dispersion theory. Simultaneously, a reduction of the temperature anisotropy takes place, mainly caused by electron heating parallel to the magnetic field. In the final stage the system reaches conditions of marginal stability which is characterized by wave packets of coherent emissions with a frequency about one half the initial value at maximum instability and such that the phase speed and group velocity coincide. These signatures are shown to relate to whistler oscillitons which are a class of nonlinear stationary waves with superimposed spatial oscillations. The calculated structures for a plasma with βe = 1.0 quantitatively resemble magnetospheric whistler events. Copyright 2007 by the American Geophysical Union.