Illuminating the Motions of Jupiter's Auroral Dawn Storms

Jupiter's auroral main emission (ME) has long been considered to be the result of currents keeping plasma corotating with the surrounding magnetosphere. As a result, the ME corotates with the planet, and individual auroral features making up the ME roughly follow suit. Jupiter's dawn storms, some of the rarest and brightest auroral features within the ME, are an exception, as they do not corotate but instead remain fixed near local dawn. The causes of this enigmatic motion are not fully understood. To test the significance of this motion, we have developed a process to identify auroral features and measure their degree of corotational motion, including dawn storms, in archival Hubble Space Telescope images of the Jovian ultraviolet aurorae. We compare motions of features inside and outside the dawn sector, characterizing the exact motions of dawn storms and providing context for these motions for the first time. In keeping with previous studies, we expected to identify features fixed near local dawn in 10% of observations; instead, we find that half of all features near local dawn lag corotation. We show that subcorotating dawn emissions are far more common than previously thought, and that the drivers of this motion must be similarly common. Corotational motion must be considered when identifying the processes driving all dawn aurorae, including the dawn storms. We explore the consistency of this result with various theories of dawn ME formation and propose that aspects of the known current system relating to the Sun-Jupiter geometry can explain this behavior.