Observations of Jupiter's Dark Polar Region from Juno-UVS and HST-STIS

The origins of the ultraviolet auroras poleward of Jupiter's main emission remains a significant unanswered question in our understanding of the dynamics of the Jovian magnetosphere. Among these polar features is the dark polar region, a region mostly devoid of emission which is typically observed in the dawn sector between the main auroral emission and the variable polar `swirl' region, and maps to the outer magnetosphere. At present it is unclear whether the dark region maps to open or closed magnetic field lines, and the location and extent of a consistent open flux region in the Jovian magnetosphere is a subject of significant debate. Here we utilise auroral observations obtained by the Juno Ultraviolet Spectrograph during the spacecraft's first 24 orbits and by the Hubble Space Telescope during concurrent imaging campaigns to analyse the morphology and evolution of the dark region over long timescales, and discuss possible mechanisms within the magnetosphere which might result in the formation of the dark polar region. Specifically, we consistently observe the extension of the dark region beyond the dawn terminator through the midnight sector, a feature not observable from Earth's orbit. We also analyse the southern dark polar region for the first time, and examine the similarities and differences between the northern and southern regions.