Jupiter boasts the largest and strongest magnetic field of any planet in the solar system, and it has spectacular displays of ultraviolet auroras to match that are caused by plasma raining down on Jupiter’s atmosphere.
However, scientists are still in the dark when it comes to deciphering the role of the solar wind on Jupiter’s auroras. The solar wind is the main factor in Earth’s auroras, with the Sun launching storms that buffet Earth’s magnetic field and inject it with plasma.
Jupiter, on the other hand, has its own plasma source: the volcanoes of its moon Io, which spew ions into space. These become trapped in Jupiter’s magnetic field, which grabs them and whips them through space in lockstep with the planet’s own rotation. This massive spinning cloud of charged particles generates a complex array of currents that course around the planet and through its ionosphere.
Previous observations show that solar storms do tend to brighten Jupiter’s auroras, but the strongest storms don’t necessarily bring the brightest auroras. Also, scientists still aren’t sure how the Sun affects the currents in Jupiter’s magnetic field.
Now new research from Kita et al. may help scientists take a big step forward in understanding the solar wind’s role. The authors used the Japanese space telescope Hisaki to monitor the strength of Jupiter’s auroras as the solar wind buffeted its magnetic field. Hisaki was designed specifically to monitor ultraviolet emission from planetary magnetic fields, and it observed Jupiter for several months between December 2013 and February 2015—a luxury that other space telescopes like Hubble don’t have.
Thanks to their long stretches of observations, the scientists noticed a correlation between the auroras and the amount of time since the last storm: The longer the quiet period was, the brighter the auroras eventually were. The authors think this means that the strength of auroras depends on how much plasma Io is able to stock up in Jupiter’s magnetosphere before a solar storm hits. A solar storm doesn’t generate auroras by the force of its impact or by supplying plasma but by tripping the system to unleash the plasma that’s already there, funneling it down to Jupiter’s atmosphere around the poles.
One of the next steps in untangling exactly how this process works will be analyzing data from NASA’s Juno spacecraft, which was launched in 2011 and reached Jupiter on 4 July 2016. The craft will get a close-up look at the planet’s polar regions to measure its magnetic field and observe the auroras. (Geophysical Research Letters, doi:10.1002/2016GL069481, 2016)
—Mark Zastrow, Freelance Writer