Nonlinear inertial and kinetic Alfven waves

Abstract

A two‐fluid (multifluid) model is used to study nonlinear kinetic (inertial) Alfven waves (solitons, oscillitons, periodic waves). The stationary wave dispersion equation yields the range of the wave speeds in which solitary waves are possible. Kinetic Alfven waves propagating almost perpendicular to the magnetic field exhibit a maximum of the phase speed (a positive dispersion followed by a negative one) which modifies the properties of solitons. At velocities greater than the maximum phase speed, soliton structure exists containing an oscillating core. Such structures, called oscillitons, also exist for other wave modes whose dispersion curves (ω − k) have an inflection point, e.g., the magnetoacoustic (whistler) mode, which gives rise to an extremum in the phase velocity. In a multifluid approach for a bi‐ion plasma, the main properties of oscillatory kinetic Alfven and magnetoacoustic waves are similar to those in a proton‐electron plasma. Assessment of kinetic effects (the Vlasov equations) shows that a maximum of the phase speed of kinetic Alfven wave remains but shifts to smaller k, and therefore the existence of oscilliton‐type solutions with embedded coherent oscillations at f ≤∼ 0.8 fp (fp is the proton gyrofrequency) is expected. In a multi‐ion Maxwellian plasma, a maximum moves further to smaller k (f ≤∼ 0.8fi, where fi is the heavy ion gyrofrequency) and even can cease when oxygen ions are injected to a proton‐electron plasma. The possibility of applying these results to observations carried out on the auroral field lines and near the magnetopause is discussed.