Journal of Plasma Physics

Papers

Magnetosonic solitons in space plasmas: dark or bright solitons?

O. A. POKHOTELOVa1, O.G. ONISHCHENKOa2, M. A. BALIKHINa1, L. STENFLOa3 and P. K. SHUKLAa3

a1 Department of Automatic Control and System Engineering, University of Sheffield, JSheffield S13JD, UK (try2comer@yahoo.co.uk)

a2 Institute of the Physics of the Earth, Russian Academy of Sciences, Moscow, Russia

a3 Department of Physics, Umea University, SE-90187 Umea, Sweden

Abstract

The nonlinear theory of large-amplitude magnetosonic (MS) waves in highβ space plasmas is revisited. It is shown that solitary waves can exist in the form of ‘bright’ or ‘dark’ solitons in which the magnetic field is increased or decreased relative to the background magnetic field. This depends on the shape of the equilibrium ion distribution function. The basic parameter that controls the nonlinear structure is the wave dispersion, which can be either positive or negative. A general dispersion relation for MS waves propagating perpendicularly to the external magnetic field in a plasma with an arbitrary velocity distribution function is derived.It takes into account general plasma equilibria, such as the Dory–Guest–Harris (DGH) or Kennel–Ashour-Abdalla (KA) loss-cone equilibria, as well as distributions with a power-law velocity dependence that can be modelled by κdistributions. It is shown that in a bi-Maxwellian plasma the dispersion is negative, i.e. the phase velocity decreases with an increase of the wavenumber. This means that the solitary solution in this case has the form of a ‘bright’ soliton with the magnetic field increased. On the contrary, in some non-Maxwellian plasmas, such as those with ring-type ion distributions or DGH plasmas, the solitary solution may have the form of a magnetic hole. The results of similar investigations based on nonlinear Hall–MHD equations are reviewed. The relevance of our theoretical results to existing satellite wave observations is outlined.

(Received January 05 2007)

(Accepted January 29 2007)