Stabilization of the fan instability: Electron flux relaxation

Abstract

This paper presents some relevant simulation results on the interaction between electrostatic waves and suprathermal electron fluxes at anomalous cyclotron and Landau resonances. In particular, the case of a dense and continuous wave spectrum is studied. It is shown that, after the waves excited by the fan instability at anomalous cyclotron resonances have reached a first saturation stage due to particle trapping, the process of “dynamical resonance merging” takes place, which leads to a strong amplification of the waves’ amplitudes. The Landau resonances do not play an essential role in the total energy exchange between the particles and the waves, as they mainly help to smooth the peaks rising during the evolution of the electron parallel velocity distribution and contribute to damping. Moreover, the paper shows that at the asymptotic stage of the interaction, when the waves’ amplitudes are saturated and the electron flux is relaxed, some physical features clearly do not fit the predictions of the well-known quasilinear theory. The careful examination of a huge number of trajectories of particles moving in the effective field of the wave packet allows to state that most of the particles involved in the resonant interactions are trapped by several waves simultaneously. In this so-called “multitrapping” process, the particles perform complex oscillatory motions which are far from what is expected from the quasilinear theory, where the diffusive behavior of the particles in the velocity space results from small successive random steps.