Excitation of the Ganymede ultraviolet aurora

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots: 1. Birkeland-type currents and associated magnetic field-aligned electric fields. 2. The stochastic heating of plasma electrons by the Landau damping of electron plasma oscillations generated by precipitated energetic electrons. We conclude that the electron density in the bright regions may attain local values as high as 10(5) cm(-3).