The mystery of Jupiter's proton aurora

The terawatts of ever-changing ultraviolet auroral emissions that are always observed with HST at both poles of Jupiter demonstrate that Jupiter's planetary system is “alive.” The characteristics of the different components of Jupiter's UV aurora provide information on the evolution of the overall state of the portion of the Jovian magnetosphere to which they connect. During the present medium-size HST campaign (HST GO-15638, cycle 26), precession of the line of apsides of Juno's orbit makes it possible to probe different regions of the magnetosphere, compared to Juno orbits during previous HST cycles. Solar wind dynamics and internal processes are known to have strong influence on Jupiter's aurora, but their relative contributions and the way they couple with each other are still under debate. Cycle 26 falls during the expected minimum of the 11-year solar activity cycle. Current measurements suggest that the solar activity is already exceptionally low, with very few solar events, like CMEs, reaching Jupiter. This provides an unprecedented opportunity to observe Jupiter's aurora during a period when its magnetosphere is mainly controlled by internal processes, therefore revealing Jupiter's natural "breathing." The present HST campaign is meant to observe Jupiter's bright FUV auroral emissions in time-tag imaging mode during Juno orbits 18 to 22 (Feb-Sep 2019). We focus on the 5-day periods prior to and during Junos perijove, when Juno is sampling the current sheet region within 60 RJ, which is expected to contain the plasma source responsible for most bright auroral components, but is in a location where these aurorae cannot be observed with Juno-UVS. We sample Jupiter's emissions at a frequency of ~1 HST visit per Jovian rotation, with typically 10 HST visits for each of the 5 Juno orbits. Here we present preliminary results inferred from HST observations and concurrent Juno in situ data, obtained during Juno orbits 18, 19 and 20.