VOYAGER-2 ULTRAVIOLET SPECTROMETER SOLAR OCCULTATIONS AT NEPTUNE - PHOTOCHEMICAL MODELING OF THE 125-165 NM LIGHTCURVES

Ingress and egress Voyager 2 ultraviolet spectrometer (UVS) solar occu ltation lightcurves at wavelengths longward of HI Lyman alpha acquired during the Neptune encounter are compared with one-dimensional methan e photochemical-transport models to infer hydrocarbon abundances and t he strength of eddy mixing in the stratosphere. Previous modeling of t he 125-140 nm lightcurves indicated eddy mixing coefficient (K) values of 3-10 x 10(6) cm(2) s(-1) near the 0.2 mu bar level and methane mix ing ratios in the lower stratosphere on the order of 1-3 x 10(-4); the se results should be insensitive to photochemical details, provided me thane is the main source of opacity at these wavelengths. The UVS ligh tcurves at the longer wavelengths, which probe beneath the CH4 photoly sis peak, are expected to be dominated by the opacity of C-2 species ( ethane, acetylene, ethylene) and perhaps higher order organics. At wav elengths > 152 nm, H-2 Rayleigh scattering is also a major opacity sou rce. Modeled C-2 species abundances are sensitive to modeling details, especially the strength and height profile of eddy mixing. The curren t photochemical model incorporates several updates, including a recent revision in CH4 photolysis branching ratios at Lyman alpha. In the ph otochemical modeling reported here, various forms for the eddy mixing profile have been tested, with the constraint that the models for egre ss conditions remain consistent with the C2H6 and C2H2 abundances near 0.5 mbar derived from IRIS measurements. Superior fits are obtained w ith models exhibiting a stagnant lower stratosphere (K approximate to 2 x 10(3) cm(2) s(-1) for pressures > 2 mbar) with a rapid transition to a localized level of vigorous eddy mixing in the upper stratosphere (K approximate to 10(8) cm(2) s(-1) near 10 mu bar, decreasing at hig her altitudes). In line with our earlier work, methane mixing ratios o n the order of 10(-4) are required to obtain good agreement between th e photochemical models and the UVS lightcurves. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.