Detailed study of FUV Jovian auroral features with the post‐COSTAR HST faint object camera

Abstract

A set of Hubble Space Telescope faint object camera images taken in the H2 bands near 1550 Å is used to infer the morphological properties of the steady state Jovian FUV aurorae. We focus on issues best addressed using the excellent spatial resolution available after correction of the spherical aberration, i.e., those related to high latitude or small auroral features. A thorough comparison of the emission loci with model ovals highlights the improvement of the VIP4 magnetic field model over previous ones at all latitudes. The north‐south conjugacy of the main oval is now correctly accounted for, and second‐order discrepancies suggest non axially symmetric contributions of external origin. This oval is usually amazingly narrow (down to 80±50 km) and very bright, although quite variable with time (100 kR to 1–2 MR, i.e., peak input flux of ∼10–200 ergs cm−2 s−1). We discuss its structure, in latitude and longitude, and show that it is consistent with precipitation by pitch angle scattering. Fainter concentric narrow 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 the north polar cap along the 160° meridian, might be not a specific auroral feature but rather a region where inner arcs and diffuse polar cap emissions are intensified, maybe by a solar wind driven ionospheric process. Equatorward of the main oval, we identify a belt of moderate emission, attributed to a precipitation process distributed between the Io torus and the distant magnetosphere. Longitudinally confined 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 confirm that the FUV footprints of the Io flux tube are very small (a few hundreds of kilometers or less), implying an interaction close to Io. The input energy flux in this spots is huge and variable (0.8–5 × 1011W).