An empirical model of the Saturn thermosphere

We present an empirical model of H-2 densities and temperatures above the homopause for the Saturn thermosphere, based on stellar occultations observed by the Cassini/UVIS instrument. These observations span the whole Cassini mission and probe a wide range of latitudes, longitudes and local times. Although the model is based on Cassini stellar occultations only, the results agree well with Voyager occultations and reasonably well with other measurements of temperature and density in the thermosphere. The principal meridional trend that the model captures is an increase in temperature with latitude away from the equator, associated with significantly elevated density level altitudes at latitudes higher than about 40 degrees in both hemispheres. These results confirm the importance of auroral heating in Saturn's thermosphere and constrain models of global circulation. We estimate the uncertainty in the model based on the standard deviation of the data points around the mean state. We do not attempt to distinguish retrieval errors from spatial and temporal trends. We find that the most important source of uncertainty is an apparent semi-annual variation in H-2 density and temperature, associated with expansion and contraction of the pressure levels in the thermosphere. Similar variation occurs in the Earth's thermosphere where it is likely driven by seasonal changes to global circulation. The model presented here is a useful tool that constrains physical models, helps to interpret existing observations, and supports the planning of future observations and missions.

Automated summary

The study presents an empirical model of H-2 densities and temperatures in the Saturn thermosphere based on Cassini/UVIS observations, which align well with other measurements. The model captures the main trend of increasing temperature with latitude in the thermosphere. The most significant source of uncertainty in the model is an apparent semi-annual variation in density and temperature.