An investigation of the temperature variations in Neptune’s upper stratosphere including a July 2008 stellar occultation event

We extracted physical atmospheric parameters from a 23 July 2008 single-chord stellar occultation of the star USNO-B1.0 0759-0739128 (I-band magnitude of 12.60) by Neptune using a light curve model fitting technique. We observed the occultation using the Agile CCD camera mounted on the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory. We found isothermal temperatures of 116.5 ± 12.0 K and 154.0 ± 13.0 K for the immersion and emersion light curve profiles, respectively. We compare the stratospheric temperatures derived from the 2008 occultation to published temperatures of Neptune at similar atmospheric pressures derived from previous stellar occultations observed in the 1980s, and from long-term photometric measurements made routinely since the 1983-1990 occultation campaign. No obvious long-term temporal variation in stratospheric isothermal temperature is present. Fluctuations in the fitted isothermal temperature values, on the order of 20 K, is evident. We explore several hypotheses to explain the observed temperature variability of Neptune’s stratosphere, including seasonal variability, variations in the Lyman-α flux received at Neptune due to the 11-year solar cycle, diurnal variations, varying insolation due to heliocentric variability, IR and UV heating by hydrocarbons, aerosol precipitation, inertia-gravity wave dissipation, and effects due to atmospheric tidal perturbations by Triton. We investigate the effects of these mechanisms on the gradual temporal changes of Neptune’s stratospheric temperature and conclude that local variations in stratospheric temperature during each event, on the order of 20 K, are dominated by viscous dissipation of inertia-gravity waves.