VOYAGER-2 ULTRAVIOLET SPECTROMETER SOLAR OCCULTATIONS AT NEPTUNE - CONSTRAINTS ON THE ABUNDANCE OF METHANE IN THE STRATOSPHERE

Voyager 2 ultraviolet spectrometer (UVS) solar occultation lightcurves at wavelengths 125-138 ran acquired during the Neptune encounter are compared with one-dimensional methane photochemical-transport models. Photoabsorption by methane (the photochemical parent) is expected to be the major source of opacity at these wavelengths, in which case the UVS lightcurves can be used to infer the strength of eddy mixing at altitudes above the methane photolysis peak and compatible CH4 mixing ratios in the lower stratosphere. For the p-T models adopted in this study, acceptable fits to the UVS lightcurves are obtained with eddy mixing coefficient values (K1/2) near the half-light altitudes (placed 550-600 km above the respective 1 bar levels) of 2-15 x 10(6) cm2 s-1 (ingress) and 4-35 X 10(6) cm2 s-1 (egress) and lower stratospheric (p > 1.0 mbar) methane mixing ratio values (f(T)(CH4)) of 5-100 x 10(-5) (ingress and egress). The ranges in f(T)(CH4) and K1/2 reflect uncertainties in the background. p-T structure and the application of different criteria for deciding what constitutes a ''good fit.'' For the nominal p-T models and a criterion based on replicating the spacing in altitude of the 125-138 nm UVS lightcurves, K1/2 values (near 0.2 mubar) of 10(7) cm2 s-1 (ingress) and 1-2 x 10(7) cm2 s-1 (egress) and f(T)(CH4) values of approximately 1 x 10(-4) (ingress and egress) are indicated, with the egress results being more uncertain. These results are insensitive to photochemical details of the models.