On the nature of the λIII brightness asymmetry in the Io torus

Abstract

The optical brightening of the Io torus around λIII∼200° has been known for 20 years, but observations have only recently revealed the nature of the asymmetry. We show that the optical asymmetry is due to changes in ion temperature, with little or no change in flux tube content. These results have significant ramifications for magnetospheric transport and torus energy flows. We examine causes of ion temperature variations and demonstrate that existing zero‐dimensional energy balance models are inadequate. We develop a more complete model of energy flow in the torus and find that small changes in energy sources result in large changes in ion and electron temperatures and densities. We extend the theory to predict brightness asymmetries at other wavelengths and show that a search for EUV asymmetries at the 30% level can help resolve the torus “energy crisis.” “Neutral Cloud Theory” predicts an EUV brightness asymmetry of exactly opposite phase to the optical asymmetry, while an unknown electron energy source around λIII∼200° could cancel or reverse this EUV asymmetry.