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

1993; American Geophysical Union; Volume: 98; Issue: A6 Linguagem: Inglês

10.1029/93ja00478

2156-2202

D. D. Barbosa,

Ionosphere and magnetosphere dynamics

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 τ D = 2 × 10 6 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 H 2 O 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 H 2 O 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.