Jupiter's magnetosphere 'acts like a giant particle accelerator'

Jupiter is a planet with some notable quirks. As well as its visually distinctive appearance, including stunning bands of clouds and the enormous storm which forms the Great Red Spot, it is also known to give off X-rays.

However, there are oddities about the X-rays it produces, which are related to its auroras. There is an open question about why the Ulysses mission, which flew past Jupiter in 1992, didn’t detect any X-rays. Now, a new paper using NASA’s NuSTAR observatory has solved this long-term mystery.

Jupiter’s southern hemisphere is shown in this image from NASA’s Juno mission. New observations by NASA’s NuSTAR reveal that auroras near both the planet’s poles emit high-energy X-rays, which are produced when accelerated particles collide with Jupiter’s atmosphere. Enhanced image by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS

NuSTAR is a space-based X-ray telescope launched in 2012, which detects high-energy X-rays which are more energetic than the kinds of X-rays typically detected by other X-ray observatories like Chandra. By investigating Jupiter with NuSTAR, researchers were able to see the highest-energy light ever detected from Jupiter, even higher energy than the planet’s X-ray auroras.

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“It’s quite challenging for planets to generate X-rays in the range that NuSTAR detects,” said lead author of the study, Kaya Mori, in a statement. “But Jupiter has an enormous magnetic field, and it’s spinning very quickly. Those two characteristics mean that the planet’s magnetosphere acts like a giant particle accelerator, and that’s what makes these higher-energy emissions possible.”

This mechanism might also explain why the Ulysses mission didn’t spot any high-energy X-rays during its flyby. The particular type of emissions caused by the interactions of electrons in the atmosphere are bright at only a particular energy level and would have been too faint for Ulysses to spot.

Even though this mystery is solved, there are still many questions about how these emissions are formed. “The discovery of these emissions does not close the case; it’s opening a new chapter,” said co-author William Dunn. “We still have so many questions about these emissions and their sources. We know that rotating magnetic fields can accelerate particles, but we don’t fully understand how they reach such high speeds at Jupiter. What fundamental processes naturally produce such energetic particles?”

The research is published in the journal Nature Astronomy.