Relative contributions of sublimation and volcanoes to Io's atmosphere inferred from its plasma interaction during solar eclipse

We present a model that describes Io's delayed electrodynamic response to a temporal change in Io's atmosphere. Our model incorporates the relevant physical processes involved in Io's atmosphere-ionosphere-magnetosphere electrodynamic interaction to predict the far-ultraviolet (FUV) radiation as Io enters Jupiter's shadow and re-emerges into sunlight. The predicted FUV brightnesses are highly nonlinear as the strength of the electrodynamic interaction depends on the ratios of ionospheric conductances to the torus Alfven conductance, but the former are functions of electrodynamics and the atmospheric density, which decays rapidly upon entering eclipse. Key factors governing the time evolution are the column density due to sublimation and the column density due to volcanoes, which maintain the background atmosphere during eclipse. The plasma interaction does not react instantaneously, but lags to a temporarily changing atmosphere. We find three qualitatively different scenarios with two of them including a post-eclipse brightening. The brightness ratio of in-sunlight/in-eclipse coupled with the existence of a sub-jovian equatorial spot constrains the volcanic column density to several times 10(18) m(-2), based on the currently available observations. Thus in sunlight, the sublimation driven part of Io's atmosphere dominates the volcanically driven contribution by roughly a factor of 10 or more. (C) 2004 Elsevier Inc. All rights reserved.