Comparative Studies of Planetary Thermospheres and Their Dynamics Using General Circulation Models
Presiding: A D Aylward, University College London; M Hickey, Embry-Riddle Aeronautical University
SA24A-01 15:30h
Magnetospheric Energy Inputs Into the Thermospheres of Jupiter and Saturn
The high temperatures observed in the upper atmospheres of the giant planets have yet to be fully explained. Joule heating and ion drag associated with magnetosphere-ionosphere coupling currents are major energy inputs at high latitudes. We use a thermospheric general circulation model to study the effects of Joule heating and ion drag upon the energetics and dynamics of the upper atmosphere. We find that the resultant temperature distribution from the mixing of magnetospheric energy and momentum inputs is more complex than initially expected.
SA24A-02 15:45h
Using HST-STIS Observations of Auroral Lyα Line Profiles to Map High-Altitude Winds on Jupiter
We present an analysis of several high-resolution long-slit echelle spectra of Jupiter's auroral Lyα emissions obtained by HST-STIS during the Cassini flyby of 12/2000-1/2001. We apply a velocity shear resonance line radiative transfer code to extract high-altitude wind velocities, using an asymmetry produced when the emergent self-reversed Lyα emission line is scattered by fast-moving H atoms high above the aurora. The neutral velocities thus extracted are complementary to the ion winds obtained by observations of Doppler-shifted auroral near-IR H3+ emissions, and provide constraints for the Jupiter Thermosphere General Circulation Model (JTGCM). The results highlight how Jupiter's aurora affect upper atmospheric dynamics on a global scale.
SA24A-03 INVITED 16:00h
Jupiter Thermospheric General Circulation Model (JTGCM): Global Structure and Dynamics Driven by Auroral and Joule Heating
A growing multi-spectral database plus recent Galileo descent measurements are being used to construct a self-consistent picture of the Jupiter thermosphere/ionosphere system. The proper characterization of Jupiter's upper atmosphere, imbedded ionosphere, and auroral features requires the examination of underlying processes including the feedbacks of energetics, neutral-ion dynamics, composition, and magnetospheric coupling. A fully 3-D Jupiter Thermospheric General Circulation Model (JTGCM) has been developed and is being exercised to address global temperatures, 3-component neutral winds, and neutral-ion specie distributions. The domain of this JTGCM extends from 20-microbar (capturing hydrocarbon cooling) to 0.1-picobar (including auroral/Joule heating processes). The resulting JTGCM has been fully spun-up and integrated for 40-60 Jupiter rotations. Results from two JTGCM cases incorporating moderate auroral heating, ion drag, and moderate to strong Joule heating processes are presented. The neutral horizontal winds at ionospheric heights vary from 0.5 km/s to 1.2 km/s, atomic hydrogen is transported equatorward, and auroral exospheric temperatures range from ~1200-1300 K to above 3000 K depending on the magnitude of Joule heating. The equatorial temperature profiles from the JTGCM are compared with the measured temperature structure from the Galileo ASI dataset. The best fit to the Galileo data implies that the major energy source for maintaining the equatorial temperatures is due to dynamical heating induced by the low latitude convergence of the high-latitude driven thermospheric circulation. The magnitude of this equatorial heating, and the strength of the underlying global thermospheric circulation, are strongly dependent upon magnetospheric coupling which regulates Joule heating. Simulated fields and diagnostics from the JTGCM are compared to available multi-spectral and spacecraft observations.
SA24A-04 INVITED 16:15h
A Saturn Thermosphere Ionosphere Model (STIM)
The theromosphere of Saturn has been modeled using the classic general circulation model (GCM) approach. A self-consistent ionosphere using photochemistry and plasma diffusion is included; the inner plasmasphere is modeled using separate analytical methods acting upon STIM output at the plasma exobase. The inclusion of ring shadowing geometery allows for the study of spatial/temporal perturbations arising from the shielding of sunlight. Applications to date include modeling the neutral temperature and electron density results obtained from the Pioneer and Voyager missions, the influence of the ionosphere upon Saturn electrostatic discharge (SED) effects for both the Voyager and Cassini epochs, and the inner plasmasphere electron densities of ionospheric orgin relevant to recent Cassini orbit insertion geometry.
SA24A-05 16:30h
Developments of STIM, the Saturn Thermosphere Ionosphere Model
The STIM (Saturn Thermosphere Ionosphere Model) model is a joint venture betwen University College London, Imperial College London, Boston University and the University of Arizona to develop a 3-d global circulation model of the Saturnian system - the primary aim being to use this as a tool for interpretation and testing of Cassini data. After initial work producing a basic thermosphere model (Muller-Wodarg et al 2005), examining issues to do with the ionosphere (Moore et al 2005) and examining auroral heating effects (Smith et al 2005), a global coupled ionosphere-plasmasphere has been added to the model. At low latitudes the model calculates ion densities on closed flux tubes passing through the ring plane. At high latitudes it performs self-consistent calculations of Joule heating and ion drag based on the calculated thermospheric and ionospheric parameters. The plasmasphere is complicated for Saturn by the strength of the centrifugal force which can dominate the forces in the outer flux tubes. Studies initially used H+ and H3+ as the principle ions but for the future it will be necessary to look at the consequences of the rings supplying OH or oxygen from ring ice particles. The high-latitude morphology is being refined as Cassini data constrains it. Long-term plans for the STIM development will be discussed.
SA24A-06 INVITED 16:45h
Gravity Wave Heating and Cooling in Saturn's Thermosphere
Recently published simulations obtained using a global circulation model demonstrate that the currently understood heat sources are unable to explain the observed temperature of Saturn's thermosphere. Those simulations provided the globally averaged altitude variation of the heat source required to reconcile observations with theory. Thus, the missing heat source, which was assumed to be due to dissipating gravity waves, was constrained by the modeling. Using this constraint and a full-wave model previously used to examine the propagation and dissipation of acoustic-gravity waves in the thermospheres of Earth and Jupiter, we simulate gravity wave propagation and dissipation in Saturn's thermosphere. A broad range of wave parameters (periods, horizontal wavelengths and energy fluxes) is considered. These simulations reveal those parts of the wave spectrum that are likely to be important to the temperature of Saturn's thermosphere.