An interpretation of the dawn-dusk asymmetry of UV emission from the Io plasma torus

Recently, the Voyager UVS experimenters made two interesting reports on the emission morphology and energetics of the Io plasma torus. First, Sandel and Broadfoot¹ provided evidence for a localized energy source in the vicinity of Io with the time-averaged power amounting to 20% of the total power radiated by the Io UV torus. Second, Shemansky and Sandel² demonstrated that the Io torus displays a local time variation in the extreme UV (EUV) emissions with a dawn-to-dusk asymmetry differing by ∼30% and that the corresponding electron energy density available for exciting the UV emissions of the oxygen and sulphur ions varies by ∼20%. These observations provide an important insight into the mass and energy flows of the Io plasma, but call into question our understanding of the physical mechanisms that maintain the radiative emissions of the heavy ions ejected from Io. Here we discuss the physical cause of the brightening near dusk side and dimming near dawn side of the Io plasma torus EUV emissions in terms of a drift shell effect. If the radial distance of the L shell of the Io plasma torus reaches a minimum around 1800 h LT but reaches a maximum around 0600 h LT, periodic adiabatic heating and cooling of the electron gas could lead to the observed modulation of the UV irradiation. Such a drift pattern of the Io torus could be produced if the plasma convection of the jovian magnetosphere were partly determined by its planetary wind outflow.