Using the kappa function to investigate hot plasma in the magnetospheres of the giant planets

Planetary magnetospheres contain two general classes of charged particles: low energy “thermal” particles (generally E < 1 keV) usually described by a Maxwellian distribution and “hot” particles ( E > 10 keV), variously called superthermal or “energetic” particles, which are described by a power law distribution. The kappa or κ function combines aspects of both Maxwellian and power law forms to provide a reasonably complete description of the particle distribution from low to high energies. Fits of the data to the kappa distribution can reveal particle density, temperature, pressure, and convection velocity, all of which are key parameters of magnetospheric physics. This paper summarizes the use of the kappa distribution and its variants to investigate the plasma properties of the magnetospheres of the giant planets Jupiter, Saturn, Uranus, and Neptune. While the κ function was used to derive convective motions in previous investigations, temperatures, and pressures at Jupiter and Saturn, it was not an optimum fit for hot plasmas at Uranus and Neptune, and in many instances may not be the optimum representation of magnetospheric plasmas.