F. Roques et al., NEPTUNE UPPER-STRATOSPHERE, 1983-1990 - GROUND-BASED STELLAR OCCULTATION OBSERVATIONS .3. TEMPERATURE PROFILES, Astronomy and astrophysics, 288(3), 1994, pp. 985-1011
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.