Thermal structure of ions and electrons in Saturn's inner magnetosphere

Abstract

A theoretical model of thermal ion and electron temperatures in Saturn's inner magnetospheres is presented. The model is based on a fast mode of radial diffusive plasma transport with a diffusion time scale which varies with distance as τD = 2 × 106 s (6/L)³. Such a model provides a conceptual organization to the energy balance problem in that the plasma residence time in the region L > 4.5 is short compared to the time scale for Coulomb energy exchange and radiative losses, whereas the opposite is true in the region L < 4.5. This condition implies that in the Dione‐Tethys plasma torus the ion and electron temperatures reflect their initial values upon creation out of the neutral H2O cloud distributed throughout the region. Oxygen ions thus have a temperature corresponding to local pickup by the magnetic field and a large perpendicular temperature anisotropy, while electrons have a temperature corresponding to that of ionization secondaries created by electron impact dissociation/ionization of H2O by the ambient hot electron population. In the collisional L < 4.5 regime the ion temperature is controlled by local pickup from O+ ‐ O charge exchange, while the electron temperature is controlled by heating from thermal O+ ions against radiative losses from electron‐excited OII. A calculation of the off‐equatorial behavior of density and temperature in the Dione‐Tethys torus based on the assumption of a constant ion temperature anisotropy along field lines is also described. The model successfully reproduces the decrease with latitude in electron and O+ temperatures which occur as a result of the interaction of electrons with the ambipolar electric potential and O+ ions with the centrifugal potential.