The longitudinal variation of the color ratio of the Jovian ultraviolet aurora: a geometric effect?

A three-dimensional model is used to assess the role of the viewing geometry on the auroral color ratio. The simulations show that both an auroral are with a geometry deduced from images obtained with the Hubble Space Telescope (HST) and a uniform polar cap emission produce no modulation or a minimum absorption when the longitude of the Jovian central meridian (CML) is close to 200 degrees. This result is in contrast with the statistical measurements made with the International Ultraviolet Explorer (IUE) spectrograph that the hydrocarbon optical depth above the auroral emission maximizes for CMLs about 180 degrees. In the frame of this simplified model, we examine a possible way to reconcile the model with the IUE data. An intrinsic longitudinal dependence of the column of methane above the level of the auroral emission is introduced in the simulation. It may result from a combination of a vigorous upwelling in sectors of strong acid stable precipitation and/or a longitudinal dependence of the characteristic energy of the auroral particles.