ON THE NATURE OF THE LAMBDA(III) BRIGHTNESS ASYMMETRY IN THE IO TORUS

The optical brightening of the Io torus around lambda(III)similar to 2 00 degrees has been known for 20 years, but observations have only rec ently revealed the nature of the asymmetry. We show that the optical a symmetry is due to changes in ion temperature, with little or no chang e in flux tube content. These results have significant ramifications f or magnetospheric transport and torus energy flows. We examine causes of ion temperature variations and demonstrate that existing zero-dimen sional energy balance models are inadequate. We develop a more complet e model of energy flow in the torus and find that small changes in ene rgy sources result in large changes in ion and electron temperatures a nd densities. We extend the theory to predict brightness asymmetries a t other wavelengths and show that a search for EUV asymmetries at the 30% level can help resolve the torus ''energy crisis.'' ''Neutral Clou d Theory'' predicts an EUV brightness asymmetry of exactly opposite ph ase to the optical asymmetry, while an unknown electron energy source around lambda(III)similar to 200 degrees could cancel or reverse this EUV asymmetry.