Abstract
Abstract. Investigating the process of electron acceleration in auroral regions, we present a study of the temporal evolution of the interaction of Alfvén waves (AW) with a plasma inhomogeneous in a direction transverse to the static magnetic field. This type of inhomogeneity is typical of the density cavities extended along the magnetic field in auroral acceleration regions. We use self-consistent Particle In Cell (PIC) simulations which are able to reproduce the full nonlinear evolution of the electromagnetic waves, as well as the trajectories of ions and electrons in phase space. Physical processes are studied down to the ion Larmor radius and electron skin depth scales. We show that the AW propagation on sharp density gradients leads to the formation of a significant parallel (to the magnetic field) electric field (E-field). It results from an electric charge separation generated on the density gradients by the polarization drift associated with the time varying AW E-field. Its amplitude may reach a few percents of the AW E-field. This parallel component accelerates electrons up to keV energies over a distance of a few hundred Debye lengths, and induces the formation of electron beams. These beams trigger electrostatic plasma instabilities which evolve toward the formation of nonlinear electrostatic structures (identified as electron holes and double layers). When the electrostatic turbulence is fully developed we show that it reduces the further wave/particle exchange. This sequence of mechanisms is analyzed with the program WHAMP, to identify the instabilities at work and wavelet analysis techniques are used to characterize the regime of energy conversions (from electromagnetic to electrostatic structures, from large to small length scales). This study elucidates a possible scenario to account for the particle acceleration and the wave dissipation in inhomogeneous plasmas. It would consist of successive phases of acceleration along the magnetic field, the development of an electrostatic turbulence, the thermalization and the heating of the plasma. Space plasma physics (charged particle motion and acceleration; numerical studies).
Highlights
The near-Earth space environment is often considered as a plasma laboratory where it is possible to investigate basic plasma processes thanks to in-situ spacecraft measurements
We describe the action of the parallel electric field on the particles
Genot et al.: Alfven waves in inhomogeneous plasmas same position range, the parallel electric field shows a succession of positive and negative peaks just below the density contour line labeled “0.67”, which corresponds to the middle of the gradient region
Summary
The near-Earth space environment is often considered as a plasma laboratory where it is possible to investigate basic plasma processes thanks to in-situ spacecraft measurements. Global ionosphere/magnetosphere electrostatic potential differences can develop over the whole auroral field lines, as in the strong double-layer model (Miura and Sato, 1980) or in the kinetic model initially proposed by Alfven and Falthammar, (1963); see Chiu and Schulz (1978) Several of those mechanisms may coexist for a given environment, direct observations of parallel electric fields suggest that they likely play an important role in particle acceleration (Andersson et al 2002a; Ergun 2001). A study of the Poynting flux associated with kinetic Alfven waves, at Freja altitude, showed that it is well directed and large enough to power locally observed accelerated electrons (Volwerk et al, 1996) The way this energy is transferred to particles is a key question which the present article proposes to address.
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