DETAILED STUDY OF FUV JOVIAN AURORAL FEATURES WITH THE POST-COSTAR HST FAINT OBJECT CAMERA

A set of Hubble Space Telescope faint object camera images taken in th e Hp bands near 1550 Angstrom is used to infer the morphological prope rties of the steady state Jovian FUV aurorae. We focus on issues best addressed using the excellent spatial resolution available after corre ction of the spherical aberration, i.e., those related to high latitud e or small auroral features. A thorough comparison of the emission loc i with model ovals highlights the improvement of the VIP4 magnetic fie ld model over previous ones at all latitudes. The north-south conjugac y of the main oval is now correctly accounted for, and second-order di screpancies suggest non axially symmetric contributions of external or igin. This oval is usually amazingly narrow (down to 80 +/- 50 km) and very bright, although quite variable with time (l00 kR to 1-2 MR, i.e ., peak input flux of similar to 10-200 ergs cm(-2) s(-1)). We discuss its structure, in latitude and longitude, and show that it is consist ent with precipitation by pitch angle scattering. Fainter concentric n arrow ovals are also present on the north polar cap, presumably at the footprint of open field lines. Both polar caps are partly covered by a faint diffuse emission, confined to the afternoon sector in magnetic local time. A bright extended feature, previously identified across t he north polar cap along the 160 degrees meridian, might be not a spec ific auroral feature but rather a region where inner arcs and diffuse polar cap emissions are intensified, maybe by a solar wind driven iono spheric process. Equatorward of the main oval, we identify a belt of m oderate emission, attributed to a precipitation process distributed be tween the Io torus and the distant magnetosphere. Longitudinally confi ned bright areas lie in the same latitude range and consist of series of short segments of concentric arcs. We also present the discovery of a narrow faint oval at the footprint of Io's orbit. Finally, we confi rm that the FUV footprints of the Io flux tube are very small (a few h undreds of kilometers or less), implying an interaction close to lo. T he input energy flux in this spots is huge and variable (0.8-5 x 10(11 ) W).