Magnetosonic solitons in space plasmas: dark or bright solitons?

摘要

The nonlinear theory of large-amplitude magnetosonic (MS) waves in high-beta 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 NIS 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 kappa-distributions. 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.