Are Dawn Storms Jupiter's Auroral Substorms?

Abstract

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 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 storms thus share many fundamental features with the auroral signatures of the substorms at Earth, despite the substantial differences between the dynamics of the magnetosphere at the two planets.Polar aurorae are a direct consequence of the dynamics of the plasma in the magnetosphere. The sources of mass and energy differ between the Earth's and Jupiter's magnetospheres, leading to fundamentally distinct auroral morphologies and very different responses to solar wind variations. Here, we report on the imaging of all development stages of spectacular auroral events at Jupiter, called dawn storms, including, for the first time, their initiation on the nightside. Our results reveal surprising similarities with auroral substorms at Earth, which are auroral events stemming from explosive magnetospheric reconfigurations. 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. Juno's observations provide the first global description of dawn storms in Jupiter's aurorae, from their initiation to their end Examples of nonisolated dawn storms and smaller events named pseudo‐dawn storms have been identified Jovian dawn storms and terrestrial auroral substorms share many morphological and temporal characteristics