Just as Earth has spectacular auroras, so too do other Solar System planets have their own versions of the atmospheric light show.
Jupiter, in fact, has the most powerful auroras in the Solar System - invisible to our eyes, but glowing brilliantly in ultraviolet wavelengths.
Because Jupiter is so wildly different from Earth, scientists are deeply invested in learning what drives these incredible atmospheric phenomena - and they just got a new clue. Thanks to the Juno orbiter, we've now observed for the first time the onset of Jupiter's mysterious auroral dawn storm.
Jupiter's auroras are produced by a constant rain of high-energy electrons mostly stripped from Io's atmosphere. These are accelerated along magnetic field lines to Jupiter's poles, where they fall into the upper atmosphere and interact with the gases to produce a glow.
This is unlike Earth's auroras, which are produced by particles from the solar wind. Also unlike Earth's auroras, Jupiter's auroras are permanent, and can behave quite differently.
One of these behaviors is the dawn storm - an intense brightening and broadening of the aurora at dawn, first observed in 1994. However, these dawn storms start on the night side of the pole, and we'd never been able to see them forming until NASA's Juno probe arrived on the scene.
"Observing Jupiter's aurora from Earth does not allow you to see beyond the limb, into the night side of Jupiter's poles," explained astronomer Bertrand Bonfond of the University of Liège in Belgium.
"Explorations by other spacecraft - Voyager, Galileo, Cassini - happened from relatively large distances and did not fly over the poles, so they could not see the complete picture. That's why the Juno data is a real game-changer, allowing us a better understanding of what is happening on the night side, where the dawn storms are born."
The emergence of a dawn storm. (NASA/JPL-Caltech/SwRI/UVS/ULiège/Bonfond)
Dawn storms are really something. They start on the night side of the planet, rotating into view as dawn breaks, transforming Jupiter's aurora into a blazing ultraviolet beacon, giving off hundreds to thousands of gigawatts of light - at least 10 times more energy than the usual Jovian aurora. They persist for a few hours before subsiding into more normal energy levels.
Because the two planets have such differences between their auroras, the process that generates the dawn storm was expected to be unlike any processes seen in Earth's auroras. Surprisingly, however, the data from Juno's ultraviolet spectrograph looked oddly familiar.
"When we looked at the whole dawn storm sequence, we couldn't help but notice that the dawn storm auroras at Jupiter are very similar to a type of terrestrial auroras called substorms," said astronomer Zhonghua Yao of the University of Liège.
Earth's auroral substorms are amazing to see. They occur when Earth's magnetosphere is disturbed by electric currents, resulting in an explosive release of energy into the ionosphere. There, the energy is dissipated as a complex, dancing aurora that can last several hours.
Substorms are strongly influenced by the solar wind and the orientation of the interplanetary magnetic field. But Earth's magnetosphere is dominated by interactions with the solar wind; Jupiter's is filled with plasma stripped from Io, which is controlled by the planet's location.
According to the team's analysis, Jupiter's auroral dawn storms are influenced by an over-spill of plasma from Io, rather than the solar wind; but the result is the same, a disturbance of the magnetosphere resulting in an explosive release of energy.
In both cases, a build-up of plasma and energy gradually increases instability in the system until boom - auroral storm.
This can only increase our understanding of the auroral processes on both planets, and could help us better understand aurora on other bodies in the future - including brown dwarfs, which have strong enough auroras to detect across interstellar space, even when they are nowhere near a star.
"Although the 'engine' of the auroras on Earth and Jupiter is very different, showing for the first time the links between the two systems allows us to identify universal phenomena and to distinguish them from the particularities relative to each planet," Yao said.
"The magnetospheres of the Earth and Jupiter store energy through very different mechanisms, but when this accumulation reaches a breaking point, the two systems release this energy explosively in a surprisingly similar way."
The research has been published in AGU Advances.