SaturnIn collaboration with colleagues across the U.S. and the U.K., we have developed a global circulation model of Saturn's upper atmosphere at Boston University. The Saturn-Thermosphere-Ionosphere-Model (STIM) calculates neutral and plasma parameters in three dimensions, and accounts for a host of processes in the Saturn system, including Joule heating, attenuation of sunlight by Saturn's rings, and neutral water influxes from Saturn's rings and icy moons. Further modifications and improvements to STIM continue to the present, as more data from the Cassini mission becomes available.
Background Neutral Parameters
Figure 1 Diurnally averaged temperatures, horizontal winds and molecular hydrogen mixing ratios versus latitude and height for equinox and solar maximum conditions, as calculated in STIM. The meridional winds (b) and zonal winds (c) are defined as positive southward and westward, respectively.
Unexpected Cooling in the Thermosphere
Figure 2 (a) Temperature structure (color contours). The dashed line separates regions to the left, which are heated by the polar energy inputs, from regions on the right, which are cooled. At latitudes smaller than those shown the temperature profile remains approximately constant to the equator, exhibiting exospheric temperatures of 150-160 K. It is thus clear that the polar energy inputs do not reproduce the observed temperature of ~400 K at 30 degrees latitude. However, the temperature of ~400 K at the pole is a good match to the infrared spectroscopic temperatures determined in this region. The solid contour represents the fixed lower-boundary temperature of 143 K. The region enclosed by this contour at ~55-70 degrees latitude is thus cooler than the lower-boundary temperature. (b) Zonal winds (color contours). The zero contour of zonal wind is shown by the solid line. Poleward of ~65 degrees latitude the winds are almost entirely westward, as expected from the direction of the ion drag. The double-lobed structure in the zonal winds is due to structures in the plasma flow model. Arros show the combined vertical and meridional circulation. The thinnest arrows represent wind speeds of < 1 meter per second, the thickness increasing linearly with the logarithm of wind speed until the thickest arrows represent wind speeds of > 100 meters per second. The cooling effect is produced form two mechanisms. First, the poleward flow induced by ion drag enhances the convective cooling of the low-latitude regions. Second, the increase in the poleward wind speed between 55 and 70 degrees latitude represents a divergence in the flow that must be balanced by upwelling to satisfy continuity. This upwelling gas expands, and cools adiabatically. It is this effect that produces temperatures cooler than the lower-boundary temperature in this region.
Global Ion Densities
Figure 3 (a) Global contours of peak electron density (Nmax) and peak height (hmax) for southern summer during solar maximum. Black areas represent portions of Saturn's ionosphere that are never photoionized. (b) Solar minimum, southern summer. (c) Solar maximum, equinox. (d) Solar minimum equinox.
Shadowing by Saturn's Rings