Magnetosphere–atmosphere coupling at Saturn: 1 – Response of thermosphere and ionosphere to steady state polar forcing

We present comprehensive calculations of the steady state response of Saturn’s coupled thermosphere–ionosphere to forcing by solar radiation, magnetospheric energetic electron precipitation and high latitude electric fields caused by sub-corotation of magnetospheric plasma. Significant additions to the physical processes calculated in our Saturn Thermosphere Ionosphere General Circulation Model (STIM–GCM) include the comprehensive and self-consistent treatment of neutral–ion dynamical coupling and the use of self-consistently calculated rates of plasma production from incident energetic electrons. Our simulations successfully reproduce the observed high latitude temperatures as well as the latitudinal variations of ionospheric peak electron densities that have been observed by the Cassini Radio Science Subsystem experiment (RSS). We find magnetospheric energy deposition to strongly control the flow of mass and energy in the high and mid-latitude thermosphere and thermospheric dynamics to play a crucial role in driving this flow, highlighting the importance of including dynamics in any high latitude energy balance studies on Saturn and other Gas Giants. By relating observed H 3 + column emissions and temperatures to the same quantities inferred from simulated atmosphere profiles we identify a potential method of better constraining the still unknown abundance of vibrationally excited H2 which strongly affects the H 3 + densities. Our calculations also suggest that local time variability in H 3 + column emission flux may be largely driven by local time changes of H 3 + densities rather than temperatures. By exploring the parameter space of possible high latitude electric field strengths and incident energetic electron fluxes, we determine the response of thermospheric polar temperatures to a range of these magnetospheric forcing parameters, illustrating that 10 keV electron fluxes of 0.1–1.2 mW m−2 in combination with electric field strengths of 80–100 mV m−1 produce H 3 + emissions consistent with observations. Our calculations highlight the importance of considering thermospheric temperatures as one of the constraints when examining the state of Saturn’s magnetosphere and its coupling to the upper atmosphere.