Illuminating the Physics of Jupiter's Auroral Dawn Storms

Jupiter's ultraviolet (UV) aurorae are the most powerful in the solar system, and are driven by a large system of currents connecting the planet's ionosphere to the most distant regions of its magnetic field-- the middle and outer magnetosphere. This internal driver makes the Jovian aurorae fundamentally different from the Earth's, which are driven externally by interaction with the solar wind. Most of Jupiter's auroral power originates in the main oval, a closed oval of emissions surrounding both magnetic poles. The currents which drive these emissions work to accelerate a constant stream of Iogenic plasma up to the same rotation rate as the planet itself. As a result, the aurorae on the main oval tend to corotate-- they rotate with the planet, remaining fixed on magnetic field lines-- and have steady brightnesses. This picture is well understood, except for the unusual and intense dawn storms which do not rotate with the planet, but instead stay fixed at dawn and have peak brightnesses ten times larger than the rest of the main oval. As these storms stay near dawn, it has been thought that they must be related to the solar wind; decades of observations and theory have not found such a connection. Here we present a statistical survey based on ~180 total hours of Hubble Space Telescope (HST) observations of the Jovian UV aurora near dawn where, rather than finding a small number of these unexplained dawn storms, we find hundreds of similar subcorotating features. These features vary in brightness from a few MegaRayleighs, as would be expected for traditional dawn storms, down to hundreds of kiloRayleighs. The physical cause of these emissions must be far more common than originally thought. With this large sample, we are able to rule out some previously theorized drivers and suggest new connections between the dawn storms, dawn aurorae as a whole, and other main oval emissions. This is a paradigm shift in understanding the dawn aurora at Jupiter and is crucial in identifying the actual driver of the dawn storms.