Stellar Occultation Saturn ' s Upper Observations of Atmosphere

We have analyzed three stellar occultation observations of Saturn's upper atmosphere during the years 1989 to 1995. These occultations probed the stratosphere at pressure levels about 0.5 to 20 pbars, intermediate to those probed by the Pioneer 11, Voyager I and II radio and ultraviolet occultations. The time span of the earlier occultations and the observation of a higher latitude occultation in 1995 can be used to detect any seasonal changes in the atmosphere, and temperature gradients between equator and higher latitudes. Based on isothermal fits, we find a temperature of 137±10 K at the latitude of 84.4 degrees on Saturn at a pressure level of 1.54±0.09 pubar. In comparison to 28 Sgr temperature, we have not detected a major seasonal change between years 1989 and 1995 at the equator of Saturn. There is also no clear indication of a dependence of the stratospheric temperature on latitude. On the other hand, our temperature results confirm the general increase of temperature from tropopause to the lower thermosphere of Saturn. The 1994 light curve exhibits a strong absorption like feature over the whole altitude range probed. Using temperature gradient models we find a lapse rate of 0.38±0.07 K km1. However the light curve could have been affected by a thin cloud layer in our atmosphere, as observations were made under minimal weather conditions. The loss of signal during this event is also attributed to the movement of the occulted star out of data frames during the event. The 1995 temperature profile covers both horizontal and vertical structure at a ratio of 8.25 to 1. This is the first temperature profile with a rather long horizontal structure of a polar region probed with a stellar occultation. The observed high-amplitude spikes in the light curve produce sudden temperature variations with amplitudes in the range 1 to 5 K. The temperature profile also exhibits a strong, local, variation of temperature of 14.5 K, over a small vertical distance of 18 km and a horizontal distance of 160 km. If the observed lapse rates in the temperature profile apply to the vertical direction only, then this region is super-adiabatic on scales of 3 to 4 km. More likely these thermal gradients are due to horizontal temperature variations. Assuming that these non-isothermal features were caused by inertia-gravity waves propagating upward in the stratosphere and breaking up in the turbulent regions, we have estimated the heating caused by viscous dissipation. Order of magnitude calculations show that this heating rate is dominant, compared to UV and IR absorptions. The same waves are also considered to be the means of transporting aerosols that were observed with HST. We also examined a propagating wave interpretation for the temperature profiles near equator, where lapse rates never reach the adiabatic value. Inertia-gravity waves with wavelengths in the order of 140 km are consistent with the equatorial data, where as north polar data suggest wavelengths less than the scale height of 44.3+4.2 Km. A paper primarily based on the 1995 data is planned for publication. Most of the data presented on this thesis will be available on the AAS CD ROM. Thesis Supervisor: James Elliot Title: Professor of Physics and Earth, Atmospheric, and Planetary Sciences