Detection of auroral hydrogen Lyman-alpha emission from Uranus.

Abstract

A series of observations of Uranus obtained with the short-wavelength spectrograph of the International Ultraviolet Explorer Observatory in 1982 April and June have revealed unexpectedly strong H Lyman-alpha emission which varied between 430 and 850 rayleighs in observed disk-averaged brightness over the course of these observations. The variability of the emission alone indicates that much of the emission must be produced by charged particle excitation of H in Uranus's upper atmosphere. In addition, comparison of these data with a model for resonant scattering of solar H Lyman-alpha emission indicates that, over a wide range of model conditions, an emission brightness of even 430 rayleighs (which was the lowest observed value) corresponds to an H column density on the order of 10 to the 17th/sq cm in Uranus's upper atmosphere. These data offer the first strong evidence for the presence of aurorae and therefore a magnetic field on Uranus.