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 occultation lightcurves at wavelengths longward of Hi Lyman α acquired during the Neptune encounter are compared with one-dimensional methane photochemical-transport models to infer hydrocarbon abundances and the strength of eddy mixing in the stratosphere. Previous modeling of the 125–;140 nm lightcurves indicated eddy mixing coefficient (K) values of 3–;10 × 106 cm2s−1 near the 0.2 μbar level and methane mixing ratios in the lower stratosphere on the order of 1–;3 × 10−4; these results should be insensitive to photochemical details, provided methane is the main source of opacity at these wavelengths. The UVS lightcurves at the longer wavelengths, which probe beneath the CH4 photolysis peak, are expected to be dominated by the opacity of C2 species (ethane, acetylene, ethylene) and perhaps higher order organics. At wavelengths > 152 nm, H2 Rayleigh scattering is also a major opacity source. Modeled C2 species abundances are sensitive to modeling details, especially the strength and height profile of eddy mixing. The current photochemical model incorporates several updates, including a recent revision in CH4 photolysis branching ratios at Lyman α. In the photochemical modeling reported here, various forms for the eddy mixing profile have been tested, with the constraint that the models for egress conditions remain consistent with the C2H6 and C2H2 abundances near 0.5 mbar derived from IRIS measurements. Superior fits are obtained with models exhibiting a stagnant lower stratosphere (K ≈ 2 × 103cm2s−1 for pressures > 2 mbar) with a rapid transition to a localized level of vigorous eddy mixing in the upper stratosphere (K ≈ 108cm2s−1 near 10 μbar, decreasing at higher altitudes). In line with our earlier work, methane mixing ratios on the order of 10−4 are required to obtain good agreement between the photochemical models and the UVS lightcurves.