Abstract. The discovery of OH in Satum's inner magnetosphere changed our view of this region from one where plasma dominated the physics to one where neutrals are dominant. We revisit Hubble Space Telescope observations of OH and derive revised OH brightnesses for observations in 1992, 1994, and 1995. These OH observations as well as Voyager observations are used as constraints on a model of neutral and plasma interactions. We find that the neutral source required to produce the observed OH brightnesses is 1.4x1027H20 s -•, with a sharp peak in the neutral source rate near 4.5 Rs. A good fit to the data requires OH densities of over 700 cm -3 at 4.5 Rs. Rapid diffusion times, about 5 days at 6 Rs, are required to match the observed ion densities. We find that the plasma and neutral composition vary with distance from Saturn, and make predictions for the ion and neutral densities as a function of radius.

[1] The origin of the large hydroxyl radical (OH) cloud near the inner moons of Saturn, indicative of a surprisingly large water-vapor source, has represented a puzzle since its discovery in 1992. A new set of Hubble Space Telescope measurements is used to constrain the OH spatial densities and to pinpoint the source region. Our model indicates that the vast majority of the water vapor (>80%) originates from Enceladus's orbital distance. This may indicate the presence of a dense population of small, as of yet unseen, bodies concentrated near Enceladus; collisions between these fragments are the suggested mechanism for producing the necessary amounts of water vapor. We show that collisions between plasma ions and neutral molecules substantially inflate the OH cloud, and increase the OH loss rate, requiring a water source three times larger than previous estimates.

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space T elescope and compare them to similar phenomena on Earth. We Ðnd that the tenuous nature of GanymedeÏs 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 ÐeldÈaligned electric Ðelds. 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 105 cm~3.