NEPTUNE UPPER-STRATOSPHERE, 1983-1990 - GROUND-BASED STELLAR OCCULTATION OBSERVATIONS .3. TEMPERATURE PROFILES

Nine ground based stellar occultations by Neptune, observed between 19 83 and 1990, provide twenty two temperature profiles of the planet upp er stratosphere, in the approximately 100-0.01 mubar region, at planet ocentric latitudes ranging from approximately 70-degrees-S to approxim ately 45-degrees-N. The temperatures derived near the 25 mubar level, where the profiles are most reliable, lie in the range 150-200 K. This pressure level is intermediate between those probed by the Voyager ra dio and UV experiments. Our results confirm the general increase of te mperature from the tropopause to the lower thermosphere of Neptune. Th ere is no clear indication of a dependence of the average stratospheri c temperature on latitude and/or insolation. On the other hand, there is evidence for a global change in the average Neptunian stratospheric temperature during this seven year time span, which could be correlat ed with the Lyman-alpha solar emission. One of the observations (June 15, 1983) exhibits absorption-like features. The higher altitude featu re (at approximately 10(-2)-10(-3) mubars) could be due to a hot Neptu nian thermosphere, or alternatively, to a faint (tau(normal) approxima tely 10(-5)) haze layer. The lower altitude feature (at approximately 50-100 mubars), is optically thicker (tau(normal) > approximately 10(- 2)), and could be connected with the haze layer detected by Voyager at about the same level (Smith et al. 1989, Sci, 246, 1422). The profile s are characterized by strong, local, variations of temperature (DELTA T approximately 30 K), over small vertical distances (approximately 30 km). Assuming that these non-isothermal features are caused by inerti a-gravity waves propagating upward in the stratosphere, we have estima ted the heating caused by viscous dissipation. Order of magnitude calc ulations show that this heating rate is dominant, compared to UV and I R absorptions by methane, in the 1-10 mubar region.