An international scientific team - including researchers from the Laboratory of Atmospheric and Planetary Physics (LPAP) at ULiège - has just uncovered the process that causes X-ray flashes to occur at the heart of Jupiter's auroras. An article published in Science Advances and based on observations obtained simultaneously by the Juno spacecraft (NASA) and the XMM-Newton telescope (ESA) lifts the veil on this mysterious phenomenon.
upiter's aurorae emit all kinds of radiation, including powerful X-ray flashes. These are all the more intriguing because they often pulse every 10 to 40 minutes. "These X-ray aurorae are caused by oxygen and sulphur ions originating from the volcanic moon Io and which, after spreading throughout Jupiter's magnetosphere, are accelerated before being precipitated into Jupiter's polar atmosphere," explains Dr Zhonghua Yao, first author of the article, a researcher at the Chinese Academy of Sciences and a scientific collaborator at the Laboratory of Atmospheric and Planetary Physics (LPAP) of the University of Liège.
"One might naively believe that observations of planets from Earth are superfluous, since we can send probes there. Nothing could be further from the truth, because the instruments that measure the particles or the magnetic field of a planet can only perform their observations from one place at a time," explains Bertrand Bonfond, FNRS Research associate at the LPAP (STAR research unit / Faculty of Science) and co-author of the study. Telescopes on the ground or in orbit around the Earth provide a global view that puts the observations of space probes, such as Juno in this case, in context. Moreover, space probes are equipped with a limited number of instruments and no X-ray camera has ever been flown on a probe to study Jupiter. "The European Space Agency (ESA) XMM-Newton telescope is therefore a valuable addition to the study of the planet," adds Prof. Denis Grodent, Director of the STAR Research Unit (Faculty of Science) and co-author of this article.
From 16 to 17 July 2017, XMM-Newton observed Jupiter continuously for nearly 26 hours and 'saw' the X-ray aurorae pulsating every 27 minutes. At the same time, Juno was approximately 4,500,000 km from Jupiter, in the heart of its magnetosphere, precisely on the magnetic field lines connected to these aurorae. By analyzing Juno's data, the researchers found a first clue: oscillations in the magnetic field with the same period as the auroral flashes. They then turned to numerical simulations of the magnetosphere to investigate the origin of these oscillations and found that they were probably generated by the friction of the solar wind on the outer layers of the magnetosphere. "That being said, these slow oscillations of the magnetic field do vibrate at the same tempo as the X-ray aurora, but they cannot accelerate the ions on their own. So there was a piece missing from the puzzle," explains Zhonghua Yao. And that is another important result of this study: this intermediate step is another phenomenon, called electromagnetic ion-cyclotron waves (also called EMIC waves). These waves are excited by the slow oscillations of the magnetic field, and they have just the right frequency to allow the ions to 'surf' on them and gain speed, until they hit the atmosphere and generate the X-ray flares.
"There is no reason why this complex interplay between waves and particles, and in particular the role of EMIC waves, should be unique to Jupiter. On Earth, EMIC waves also accelerate protons in the auroral atmosphere," says Zhonghua Yao. This is a fundamental process that applies to Saturn, Uranus, Neptune and probably also to exoplanets. Beyond the planets, there are larger environments where this process could also occur. In the study of galaxy clusters, astronomers are finding that the gases flowing between the clusters have similar densities, temperatures and other properties to those of Jupiter's magnetosphere. It is therefore possible that suh electromagnetic waves also play an important role in transferring energy from one place to another.
Jupiter's mysterious X-ray aurora has been explained, ending a 40-year quest for an answer. For the first time, astronomers have seen how Jupiter's magnetic field is compressed, which heats up particles and directs them along the magnetic field lines into Jupiter's atmosphere, triggering the X-ray auroras. The link was made by combining in situ data from NASA's Juno mission with X-ray observations from ESA's XMM-Newton telescope.
Z.H. Yao et al, Revealing the source of Jupiter's x-ray auroral flares, Science Advances, 2021 DOI: 10.1126/sciadv.abf0851