Results from the occultation of the sun by Neptune imply a temperature of 750 +/- 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 +/- 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 +/- 5 kelvins, and the surface pressure is roughly 14 microbars.

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Occultation observations of the upper atmosphere of Uranus by the Voyager 2 ultraviolet spectrometer are analyzed. The measurements extend from 0.5 mbar to about 10 -6 /•bar using the EUV wavelengths 520 _< • _< 1700 •. H 2 dominates the atmosphere (the approximately 15% He content deduced by the Voyager 2 infrared spectrometer cannot be seen in occultation at these wavelengths) up to the vicinity of the exobase near 1.25 R v, where atomic H becomes the major constituent. Apparently because of weak eddy mixing, the hydrocarbon mixing ratios are quite small in the measured pressure range, so that the atmosphere is more transparent than those of Jupiter and Saturn. Thus H 2 Rayleigh scattering is the dominant source of opacity in the lower portion of the observed pressure range. The mixing ratio of C2H 2 is on the order of 10 -8 there, while only an upper limit (_< 10 -7) is available for CH,•. Also, some evidence exists for the possible presence of C2H 6 at a mixing ratio of several x 10 -8 The value of the eddy diffusion coefficient at the homopause is much lower than at Jupiter and Saturn' the best fitting of several photochemical models which were matched to the observations assumed a value of 10 '• cm 2 s-x. This may represent an upper limit. In addition, the two high-latitude occultations indicate little difference in upper atmospheric structure between the day and night hemispheres, despite the constancy of the illumination geometry over recent decades. The atmospheric temperature above about 0.01 to 0.001/•bar is 800 ñ 100 K. Because of this high temperature the thermal component of the H exosphere extends to great altitude, with number densities of several hundred cm-3 at 2 R v. This high gas density has important implications for ring dynamics, possibly being responsible for the extreme narrowness and isolation of the visib, le Uranian rings. The extent and density of the H exosphere and the nonthermal corona (which has an even larger scale height) will also strongly affect the origin and maintenance of the unusual plasma populations observed at Uranus by the Voyager 2 plasma science and low-energy charged particle experiments.

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 10(29) per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 10(20) per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x10(11) watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10(-7) at a vertical column abundance of molecular hydrogen of 10(23) per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

[1] Resolution of the details of a planetary magnetic field from magnetometer measurements made during a single flyby can be severely limited because of the incomplete geometrical sampling of the planetary neighborhood by the flyby trajectory. This problem was especially severe for the only spacecraft encounter with Uranus, that of Voyager 2 in 1986. Fortunately, auroras at the magnetic field line footprints serve as additional constraints that may be used to determine the higher multipole moments of planetary fields (Connerney et al.'s (1998) VIP-4 model of for Jupiter). In the present work, this approach is applied to improving the resolution of the magnetic field of Uranus. The auroral emission distribution at Uranus is determined from scans by the Voyager 2 Ultraviolet Spectrometer (UVS), enhancing an earlier analysis by Herbert and Sandel (1994) by incorporating more observations and by using more powerful analysis techniques. The resulting new determination of the auroral ovals is well correlated with the field lines associated with the strongest plasma wave and radio emissions but differs from model ovals computed by Connerney et al. (1987) from the Q 3 magnetic field model for Uranus. Consequently, a search has been initiated for model coefficients of the planetary magnetic field that agree both with the magnetic field observations and also with the reasonable assumption that the newly determined auroral emissions lie at the magnetic footprints of an equidistant circum-Uranian region of the magnetosphere. The dipole and quadrupole terms of the new field model, termed AH 5 , are similar to those of the dipole + quadrupole Q 3 model, but the AH 5 higher multipole terms diverge from the dipole + quadrupole + octupole I3E1 model of Connerney et al. (1987), from which the Q 3 model was derived. Inasmuch as the I3E1 octupole terms were not resolved, the AH 5 model derived here comprises a first estimate of the higher multipole moments of Uranus's magnetic field.