Modeling of global variations and ring shadowing in Saturn's ionosphere

A time-dependent one-dimensional model Of Saturn's ionosphere has been developed as all intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used (H-divided by . H-2(divided by). H-3(divided by), anti He+) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. occultation data front the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions. both with and without shadowing, from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium. whereas, diffusive processes are important in the topside. In agreement with previous I-D models. the ionosphere is dominated by H-divided by and H-3(divided by), with a Peak electron density of similar to 10(4) electrons cut At low- and mid-latitudes, H-divided by is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [H+]/[H-3(+)] decreases, with H-3(+) becoming, the dominant ion at altitudes near the peak (similar to1200-1600 km) for noon-tin-le hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice. the season for the Cassini spacecraft's encounter with Saturn. attenuation has a maximum of two orders of magnitude. Causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of H-2. with vibrationally excited H-2. and for different influxes of H2O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be similar to 42 mho in the summer hemisphere during solstice and similar to 18 mho during equinox. indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable. (C) 2004 Elsevier Inc. All rights reserved.