Dawn storms are among the brightest events in the Jovian aurorae. Up to now, they had only been observed from Earth-based observatories, only showing the Sun-facing side of the planet. Here we show for the first time global views of the phenomenon, from its initiation to its end and from the nightside of the aurora onto the dayside. Based on Juno's first 20 orbits, some patterns now emerge. Small short-lived spots are often seen for a couple of hours before the main emission starts to brighten and evolve from a straight arc to a more irregular one in the midnight sector. As the whole feature rotates dawn-ward, the arc then separates into two arcs with a central initially void region that is progressively filled with emissions. A gap in longitude then often forms before the whole feature dims. Finally, it transforms into an equatorward-moving patch of auroral emissions associated with plasma injection signatures. Some dawn storms remain weak and never fully develop. We also found cases of successive dawn storms within a few hours. Dawn storm thus share many fundamental features with the auroral signatures of the substorms at Earth. These findings demonstrate that, whatever their sources, mass and energy do not always circulate smoothly in planetary magnetospheres. Instead they often accumulate until the magnetospheres reconfigure and generate substorm-like responses in the planetary aurorae, although the temporal and spatial scales are different for different planets.

Since 2016, the Juno-UVS instrument has been taking spectral images of Jupiter’s auroras during its polar fly-bys. These observations provide a great opportunity to study Jupiter’s auroras in their full extent, including the nightside, which is inaccessible from Earth. We present a systematic analysis of features in Jupiter’s polar auroras called auroral bright spots observed during the first 25 Juno orbits. Bright spots were identified in 16 perijoves (PJ) out of 24 (there was no available data for perijove 2), in both the northern and southern hemispheres. The emitted power of the bright spots is time variable with peak power ranging from a few tens to a hundred of gigawatts. Moreover, we found that, for some perijoves, bright spots exhibit quasiperiodic behavior. The spots, within PJ4 and PJ16, each reappeared at almost the same system III position of their first appearance with periods of 28 and 22 minutes, respectively. This period is similar to that of quasiperiodic emissions previously identified in X-rays and various other observations. The bright spot position is in a specific region in the northern hemisphere in system III, but are scattered around the magnetic pole in the southern hemisphere, near the edge of the swirl region. Furthermore, our analysis shows that the bright spots can be seen at any local time, rather than being confined to the noon sector as previously thought based on biased observations. This suggests that the bright spots might not be firmly connected to the noon facing magnetospheric cusp processes.

Auroral brightness and color ratio imagery, captured using the Juno mission’s Ultraviolet Spectrograph, display intense emissions poleward of Jupiter’s northern main emission, and these are split into two distinctly different spectral or “color ratio” regimes. The most poleward region, designated the “swirl region” by Grodent et al. (2003), exhibits a high color ratio, while low color ratio emissions are found within the collar around the swirl region but still poleward of the main emission. We confirm the apparent strong magnetospheric local time control within the polar collar (Grodent et al., 2003), with the dusk side bright “active region” emissions extending from ~11 to 22 hr of magnetospheric local time. These bright emissions dim by at least an order of magnitude between ~0 and 11 hr magnetospheric local time, in the midnight to dawn side “dark region”. This magnetospheric local time structure holds true even when the entire northern oval is located on the night side of the planet (in ionospheric local time), a geometry unstudied prior to Juno, as it is unobservable from Earth. The swirl region brightens at ionospheric dawn (~5-7 ionospheric local time) and diminishes or completely disappears at ionospheric local times of ~20 to 22 hrs. Finally, the southern auroral polar emissions appear to share all of the local time dependencies of its northern counterpart, but at a reduced intensity.