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 View the MathML sourceH3+ 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 View the MathML sourceH3+ densities. Our calculations also suggest that local time variability in View the MathML sourceH3+ column emission flux may be largely driven by local time changes of View the MathML sourceH3+ 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 View the MathML sourceH3+ 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

Upper atmospheres and ionospheres of planets and satellites

The upper atmospheres of the planets and their satellites are more directly exposed to sunlight and solar wind particles than the surface or the deeper atmospheric layers. At the altitudes where the associated energy is deposited, the atmospheres may become ionized and are referred to as ionospheres. The details of the photon and particle interactions with the upper atmosphere depend strongly on whether the object has anintrinsic magnetic field that may channel the precipitating particles into the atmosphere or drive the atmospheric gas out to space. Important implications of these interactions include atmospheric loss over diverse timescales, photochemistry and the formation of aerosols, which affect the evolution, composition and remote sensing of the planets (satellites). The upper atmosphere connects the planet (satellite) bulk composition to the near-planet (-satellite) environment. Understanding the relevant physics and chemistry provides insight to the past and future conditions of these objects, which is critical for understanding their evolution. This chapter introduces the basic concepts of upper atmospheres and ionospheres in our solar system, and discusses aspects of their neutral and ion composition, wind dynamics and energy budget. This knowledge is key to putting in context the observations of upper atmospheres and haze on exoplanets, and to devise a theory that explains exoplanet demographics.Comment: Invited Revie

Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS

We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside