Neptune's upper stratosphere, 1983-1990: ground-based stellar occultation observations III. Temperature profiles
Abstract
Nine ground based stellar occultations by Neptune, observed between 1983 and 1990, provide twenty two temperature profiles of the planet upper stratosphere, in the ~100-0.01μbar region, at planetocentric latitudes ranging from ~70 deg S to ~45 deg N. The temperatures derived near the 25μbar 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 radio and UV experiments. Our results confirm the general increase of temperature from the tropopause to the lower thermosphere of Neptune. There is no clear indication of a dependence of the average stratospheric 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 correlated with the Lyman-α solar emission. One of the observations (June 15, 1983) exhibits absorption-like features. The higher altitude feature (at ~10^-2^-10^-3mubars) could be due to a hot Neptunian thermosphere, or alternatively, to a faint (τ_normal_~10^-5^) haze layer. The lower altitude feature (at ~50-100μbars), is optically thicker (τ_normal_>~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 profiles are characterized by strong, local, variations of temperature ({DELTA}T~30 K), over small vertical distances (~30km). Assuming that these non-isothermal features are caused by inertia-gravity waves propagating upward in the stratosphere, 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 by methane, in the 1-10μbar region.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- August 1994
- Bibcode:
- 1994A&A...288..985R
- Keywords:
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- NEPTUNE;
- OCCULTATIONS