Our simulations originate from two codes developed side-by-side but separately, namely, the Saturn Thermosphere GCM (Müller-Wodarg et al., 2006) and Saturn 1-D Ionosphere Model (Moore et al., 2004) which were subsequently fully coupled to form the Saturn Thermosphere Ionosphere Model (STIM). The thermosphere component globally solves the non-linear Navier–Stokes equations of momentum, continuity and energy on a spherical pressure level grid. The momentum equation includes terms such as pressure gradients, viscous drag, Coriolis acceleration, curvature accelerations and advection. The energy equation includes all processes of internal energy redistribution, such as advection, adiabatic heating and cooling as well as molecular and turbulent conduction. Solar EUV heating is calculated through explicit line-of-sight integration of solar irradiance attenuation (the Lambert–Beer Law), assuming solar spectra derived from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/Solar EUV Experiment (SEE) (Woods et al., 2005; Woods, 2008) and heating efficiencies of 50%, a value in agreement with estimates for Jupiter by Waite et al. (1983). While we include direct solar EUV heating in our calculations, it has a negligible influence on the energy balance of Saturn’s thermosphere, as shown earlier by Müller-Wodarg et al. (2006). We show in Section 3 that the main importance of solar EUV radiation lies in its ionising role that leads to conductivities, Joule heating and ion drag which in turn affect the thermospheric energy budget and dynamics.
Solar EUV heating
heating efficiencies
50%
%