Voyager ultraviolet stellar occultation measurements of the composition and thermal profiles of the Saturnian upper atmosphere

Abstract

Occultation of the star 3-Scorpii by Saturn as recorded by the Voyager Ultraviolet Spectrometer yields the value of the exospheric temperature in the equatorial region to be 800 (+150,-120) K at an altitude of 1540 km above the 1-bar atmospheric pressure level; the He-density at 1540 km is determined to be 5 (+3.6, -1.8) x 109 cm -3. Temperature gradient in the thermosphere is found to be 1.25 (+0.05, -0.07) K km -1. Methane volume mixing ratio at 965 km above the 1-bar pressure level is determined to be 1.5 x 10 -4.

Polar heating in Saturn's thermosphere

“…A recent reanalysis of the Voyager occultations (Vervack et al, 2005 1 ) suggests that T ∞ at 30 • latitude may be closer to ∼550 K. However, for the purposes of this study we will continue to consider the published results of Smith et al (1983) and Festou and Atreya (1982). We will, however, discuss the implications of the reanalysis upon our results.…”

“…The horizontal band in Fig. 6 represents the 800 K at 4 • N measurement of Festou and Atreya (1982) in the same way. The vertical bands on both of Figs.…”

“…To compare the available observations with the model-derived temperatures it is, of course, necessary to associate each measurement with specific grid points within the model. In the case of the spacecraft occultation data we associate the measured exospheric temperatures of 400 K at 30 • N (Smith et al, 1983) and 800 K at 4 • N (Festou and Atreya, 1982) with the model-derived temperatures at the topmost pressure level and at the appropriate latitude. In the case of the ground-based spectroscopic measurements we assume that the observed emission is associated with the peak of the ionosphere, and that this in turn is, approximately, spatially coincident with the peak of q h .…”

“…We find that the form of the topside heating profile is particularly important in determining exospheric temperatures. We compare our results to exospheric temperatures from Voyager occultation measurements (Smith et al, 1983;Festou and Atreya, 1982) and auroral H + 3 temperatures from ground-based spectroscopic observations (e.g. Miller et al, 2000).…”

“…Smith (chriss@apl.ucl.ac.uk) performed by the Voyager spacecraft (Smith et al, 1983;Festou and Atreya, 1982) and ground-based infrared spectroscopic measurements of H + 3 temperatures in the polar regions of the planet (Miller et al, 2000). The Voyager solar occultation experiment (Smith et al, 1983) returned an exospheric temperature of T ∞ ∼420 K at a latitude of 30 • N. In contrast, the stellar occultation experiment (Festou and Atreya, 1982) gave a value of T ∞ ∼800 K at 4 • N. In the decades since Voyager there has been considerable debate over the relative validity of these results. For a detailed analysis of the advantages and disadvantages of occultation measurements, and a discussion of the Smith et al (1983) and Festou and Atreya (1982) measurements at Saturn, we refer the reader to the excellent review by Smith and Hunten (1990).…”