Abstract
Abstract. The propagation of dispersive Alfvén waves in a low-beta collisionless plasma with a high-density channel aligned with the ambient magnetic field, is studied in three space dimensions. A fluid model retaining linear Landau damping and finite Larmor radius corrections is used, together with a hybrid particle-in-cell simulation aimed to validate the predictions of this Landau-fluid model. It is shown that when the density enhancement is moderate (depending on the pump wavelength and the plasma parameters), the wave energy concentrates into a filament whose transverse size is prescribed by the dimension of the channel. In contrast, in the case of a stronger density perturbation, the early formation of a magnetic filament is followed by the onset of thin helical ribbons and the development of strong gradients. This "dispersive phase mixing" provides a mechanism permitting dissipation processes (not included in the present model) to act and heat the plasma.
Highlights
The phenomenon of Alfven wave filamentation is viewed as one of the possible mechanisms at the origin of the magnetic tubes observed in the magnetosheath (Alexandrova et al, 2004, 2006)
The propagation of dispersive Alfven waves in a low-beta collisionless plasma with a high-density channel aligned with the ambient magnetic field, is studied in three space dimensions
The present paper concentrates on the formation of magnetic filaments by focusing of a monochromatic Alfven wave in the presence of a density channel aligned with the ambient magnetic field, and the later transition to a regime of dispersive phase mixing when the initial density enhancement is strong enough
Summary
The phenomenon of Alfven wave filamentation is viewed as one of the possible mechanisms at the origin of the magnetic tubes observed in the magnetosheath (Alexandrova et al, 2004, 2006). The problem arises of the effect of density inhomogeneities on Alfven wave filamentation, i.e. in an intermediate regime where dispersion is important and inhomogeneities not strong enough for the usual phase mixing mechanism to dominate It has been shown in the Hall-MHD framework (Borgogno et al, 2008) that the conditions for the formation of Alfven filaments can be significantly broadened in the presence of high or low density channels of relatively small amplitude (possibly randomly distributed) aligned with the ambient field.
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