The Voyager 2 photopolarimeter experiment measured the ultraviolet flux from the star &ggr; Pegasi as the star emerged from behind the Uranus limb at planetocentric latitude 68.9¿ N on the nightside. We have compared these measurements to model atmospheres with, and without, an aerosol component. If the atmosphere is free of aerosols, the temperature is 85¿2.3 K at a pressure of 2.7 mbar, 90¿6 K at 1 mbar, and 96¿13 K at 0.37 mbar, and the radius of the 1-mbar level is 25,219¿6.3 km. The radius value is close to the inferred radius (25,225 km) of the 1-mbar level at latitude 69¿ in the southern hemisphere (G. Lindal, private communication, 1987). This agreement argues for hemispheric symmetry in the shape of the 1-mbar geopotential and, therefore, in the latitude profile of the zonal wind velocity which was not measured in the northern hemisphere. Our measurement, when coupled with a 1-mbar equatorial radius of 25,734 km (G. Lindal, private communication, 1986), leads to an effective planetary oblateness of &egr;=0.0230¿0.0004, slightly smaller than the value (0.024) determined from earlier work.

We place upper limits to the amount of aerosol that can form a discrete layer or haze-top boundary; the aerosol extinction coefficient, kext≤~10-4 km-1, if the layer is at an altitude equal to or higher than the 1-mbar altitude. Our ability to sense haze boundaries diminishes rapidly below 3 mbar. A well-mixed haze may be present. Candidates for this haze include dust from meteor and ring particle infall, photochemically formed diacetylene condensing at temperatures below 103 K, and other photochemical species which condense at temperatures higher than about 80 K. We show that dust from the rings and meteors contributes no more than a few tenths of a percent to the extinction profile. Current predictions for the diacetylene abundance reveal that diacetylene aerosols may contribute as much as a few tens of percent to the extinction profile and can be accommodated by our observations. More detailed photochemical-transport-aerosol models are needed to explore these questions further. ¿ American Geophysical Union 1987