Alfvén wave filamentation and dispersive phase mixing

Abstract

The formation of three‐dimensional magnetic structures from quasi‐monochromatic left‐hand polarized dispersive Alfven waves, under the effect of transverse collapse and/or the lensing effect of density channels aligned with the ambient magnetic field is discussed, both in the context of the usual Hall‐MHD and using a fluid model retaining linear Landau damping and finite Larmor radius corrections. It is in particular shown that in a small‐β plasma (that is stable relatively to the filamentation instability in the absence of inhomogeneities), a moderate density enhancement leads the wave energy to concentrate into a filament whose transverse size is prescribed by the dimension of the channel, while for a strong density perturbation, this structure later on evolves to thin helical ribbons where the strong gradients permit dissipation processes to become efficient and heat the plasma. The outcome of this “dispersive phase mixing” that leads to small‐scale formation on relatively extended regions contrasts with the more localized oblique shocks formed in the absence of dispersion. Preliminary results on the effect of weak collisions that lead to an increase of the transverse ion temperature are also briefly mentioned.