Particle-In-Cell simulations of circularly polarised Alfvén wave phase mixing: A new mechanism for electron acceleration in collisionless plasmas

Abstract

Abstract. In this work we used Particle-In-Cell simulations to study the interaction of circularly polarised Alfvén waves with one dimensional plasma density inhomogeneities transverse to the uniform magnetic field (phase mixing) in collisionless plasmas. In our preliminary work we reported discovery of a new electron acceleration mechanism, in which progressive distortion of the Alfvén wave front, due to the differences in local Alfvén speed, generates an oblique (nearly parallel to the magnetic field) electrostatic field. The latter accelerates electrons through the Landau resonance. Here we report a detailed study of this novel mechanism, including: (i) analysis of broadening of the ion distribution function due to the presence of Alfvén waves; and (ii) the generation of compressive perturbations due to both weak non-linearity and plasma density inhomogeneity. The amplitude decay law in the inhomogeneous regions, in the kinetic regime, is demonstrated to be the same as in the MHD approximation described by Heyvaerts & Priest (1983, A&A, 117, 220).

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

“…We can see that for a weak ion to electron mass ratio (16 in the work by Tsiklauri et al [2005]) this perturbation is strong, while for a larger mass ratio (400 in the present study), the ion velocity perturbation is lower. Therefore, we can conclude that there is no significant ion acceleration in the present simulations, and this result agrees with the analysis conducted by other authors.…”

“…Therefore, the difference of time spent by the upgoing and the downgoing ions in the acceleration region (that moves with the waves) is weak compared to the case of the electrons, and the upgoing and the downgoing ion accelerations have approximately the same efficiency. Tsiklauri et al [2005] made a series of simulations, in the context of Solar physics, that presents similarities with the simulations presented here and in previous papers (see Mottez et al [2006] for a detailed comparison). In the work by Tsiklauri et al [2005], plots of the ion distribution functions show a broadening that is not seen in our simulations.…”

“…Tsiklauri et al [2005] made a series of simulations, in the context of Solar physics, that presents similarities with the simulations presented here and in previous papers (see Mottez et al [2006] for a detailed comparison). In the work by Tsiklauri et al [2005], plots of the ion distribution functions show a broadening that is not seen in our simulations. In Run A for instance, we can see only an increase by 5% of the ion perpendicular thermal speed.…”

“…More recently a comparative study conducted with FAST data [Chaston et al, 2006] revealed that predictions of the present model were actually observed (electron acceleration on the density gradients, wave focusing in the cavity, ..). On the simulation side, analog results were found independently by Tsiklauri et al [2005]. However these simulations were initialized with a sinusoidal AW (one wavelength covering the parallel length of the simulation domain), whereas observed SKAW propagate as isolated wave packets (see for instance observations by the Freja spacecraft in the work by Louarn et al [1994] and Volwerk et al [1996]).…”

“…In Run A for instance, we can see only an increase by 5% of the ion perpendicular thermal speed. Actually, when Tsiklauri et al [2005] analyzed the contribution of this broadening to the energy budget, they found no contribution, because the kinetic energies in the directions y and z oscillate in antiphase. Therefore, they conclude that there is no ion acceleration in their simulations and that the ion distribution broadening is due to the usual velocity perturbations associated to the AW.…”