IUE OBSERVATIONS OF NE 'ff!UNE FOR H LYMAN-c• EMISSION

Neptune has been observed on seven occasions with the International Ultraviolet Explorer Observatory (IUE, Boggess et al. 1978) in an attempt o detect planetary H Ly-a line emission. The observing technique is the same as previously employed in IUE observations of Uranus, described in detail by Clarke et al. (1986), and consists of spatial separation of the planetary emission from background geocoronal and interplanetary H Ly-a emissions within the spectrograph aperture. No emission has been detected from Neptune in any of the observations, with 1 a upper limits to the planet-averaged surface brightness as low as !80 Rayleighs. Despite poorer sensitivity in observing Neptune from Earth orbit compared to the other planets, the inwinsic brightness is significantly less than the 400-1500 R that would be expected from scaling arguments. This Upper limit does not rule out scattered solar H Ly-a emission from a Jupiter-like atmosphere, and it does not rule out auroral emission from an active magnetosphere. The main significance is for the efficiency of the electroglow process on Neptune, and Neptune is significantly less efficient at producing bright aurora and/or electroglow than Uranus. This may further indicate that Neptune has a lower upper atmospheric temperature than Uranus.