Aurora and airglow on the night side of Neptune



We have examined the latitude-longitude distribution of emissions detected by the Voyager ultraviolet spectrometer on the dark hemisphere of Neptune, The emissions have two significant geographic features: (1) a broad peak near longitude 60° W that extends rather uniformly over the entire range of observed latitudes (55° S to 50° N); and (2) a brighter, narrower peak near the south and 240° W. We interpret the first peak as due to excitation of the night side atmosphere by photoelectrons from the magnetically conjugate, sunlit atmosphere. The second peak can plausibly be attributed to a southern aurora; the field geometry would then seem to require a conjugate (and probably brighter) northern aurora that escaped detection poleward of the latitude range sampled by the UVS data. The power for such an aurora could be extracted from Neptune's rotation by the injection of plasma at Triton's orbit at a rate dm/dt ∼ 1 kg/s.