Io's auroral limb glow: Hubble Space Telescope FUV observations

Observations of Io's FUV aurora were obtained using the Space Telescope Imaging Spectrograph (STIS) on several occasions between 1997 and 2000. The STIS first‐order long‐slit spectroscopy mode and 2″ wide slit were used to produce nearly monochromatic images of Io. These images reveal several distinct auroral features, including limb brightened emissions at the poles, designated “limb glow”. A detailed study of the limb glow brightness in OI] 1356 Å images is reported. The limb glow on the hemisphere facing the plasma torus centrifugal equator is consistently brighter than on the other hemisphere, and is brighter by a factor of ∼2 when Io is farthest from the plasma torus centrifugal equator. We determine that this behavior is consistent with there being more electrons and more electron energy in the part of an intersecting plasma torus flux tube located above the brighter hemisphere. Since most of the electrons in an intersecting flux tube have time to travel along the field line and collide with Io before the flux tube moves downstream across Io's poles, more torus electron energy is transferred into the polar hemisphere with brighter aurora. We demonstrate that a Kappa distribution model of the plasma torus electron column density above each hemisphere predicts to first order the ratio of north and south limb glow brightnesses as a function of Io's distance from the plasma torus centrifugal equator. This finding illustrates the importance of field‐aligned torus electron energy transport for producing Io's aurora.