Ionospheric contribution to Saturn's inner plasmasphere

Ion densities from the three-dimensional Saturn thermosphere-ionosphere model are extended above the plasma exobase using the formalism of Pierrard and Lemaire ( 1996) which evaluates the balance of gravitational, centrifugal, and electric forces on plasma in a rotating dipole field. The parameter space of ionospheric contributions to Saturn's inner plasmasphere (to within 1 Saturnian radius above the limb, L < 2) is explored by comparing results that span the observed extremes of plasma temperature, 650-1700 K, and the range of velocity distribution functions, Lorentzian (or Kappa, with kappa = 2) to Maxwellian (i. e., a Lorentzian with kappa = infinity). Plasmaspheric densities are generated for solstice and equinox conditions using solar maximum and solar minimum solar fluxes. Magnetic flux tubes in Saturn's inner plasmasphere are modeled to have less plasma above the exobase (similar to 4000 km) than below it in contrast to the terrestrial situation. Calculations are made for plasma densities along the path of the Cassini spacecraft's orbital insertion on 1 July 2004. The full range of modeled electron density at closest approach is 0.02-220 cm(-3), although increased knowledge of either plasma temperature or velocity distribution at Saturn's plasma exobase would tighten this range considerably. There is reason to favor the higher values of the calculated electron density as the plasma population at Saturn's plasma exobase is more likely to exhibit a non-Maxwellian velocity distribution. There is a wide range of combinations of k and plasma temperature that leads to predictions of order 100 el cm(-3) at Cassini's closest approach, 1.3 R-S.