Kinetic Alfvén solitary waves in a plasma with two-temperature superthermal electron populations: the case of Saturn’s magnetosphere

Thanks to the evidence provided by the Cassini spacecraft mission, it is now established that
Saturn’s magnetospheric plasma consists of various types of positive ions, as well as two
distinct populations of electrons, at different temperatures. The electron population energy
distributions are characterized by long suprathermal tails and have been effectively modelled
by kappa-type distributions. Plasma properties are known to vary along the radial direction.
A strong magnetic field penetrates the magnetosphere, hence the plasma beta is small, β <
1 for radial distance < 15.2RS (where RS = 60 330 km is the Saturn’s radius). Motivated
by these observations, we have investigated the conditions for existence and the dynamics
of linear and non-linear kinetic Alfv´en waves (KAWs) in Saturn’s magnetosphere. We have
considered a low-β (stronglymagnetized) plasma, comprising of positive ions and two electron
populations (‘cold’ and ‘hot’) characterized by non-Maxwellian (kappa) distributions. In the
small-amplitude regime, harmonic analysis leads to a linear dispersion relation bearing explicit
dependence on the characteristics of the suprathermal components. In the nonlinear regime,
large-amplitude stationary profile kinetic Alfv´en solitary wave solutions are obtained via a
two-component pseudopotential method, associated with either positive or negative potential
structures (pulses) propagating at sub- and super-Alfv´enic speeds, respectively. The effect of
various intrinsic plasma configuration properties (hot-to-cold electron density and temperature
ratio; superthermality indices κc and κh; plasma beta) as well as propagation parameters (pulse
speed, direction of propagation) on the characteristics of KAW solitary waves are discussed