The 40-year-old mystery of what causes Jupiter’s X-ray auroras has been solved. For the 1st time, astronomers have seen the whole mechanism at work – and it might be a process occurring in many other parts of the Universe too.
Planetary astronomers have studied Jupiter’s spectacular X-ray auroral emission for many years . The X-ray ‘colours’ of those auroras show that they’re triggered by electrically charged particles called ions crashing into Jupiter atmosphere. But astronomers had no idea how the ions were ready to get to the atmosphere within the first place.
Now, for the 1st time, they need seen the ions ‘surfing’ electromagnetic waves in Jupiter’s magnetic flux , down into the atmosphere.
The vital clues came from a latest analysis of data’ from ESA’s XMM-Newton telescope & NASA’s Juno spacecraft. Situated in Earth’s orbit, XMM-Newton makes remote observations of Jupiter at X-ray wavelengths. Juno on the opposite hand circles the enormous planet itself, taking in-situ readings from inside Jupiter magnetic flux . But the question was: what should the team look for?
The clue came when Zhonghua Yao, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, and lead author of latest study, realised that something didn’t add up about Jupiter’s X-ray auroras.
On Earth, auroras are visible only in belt surrounding the magnetic poles, between 65-80 degrees latitude. Beyond 80 degrees, auroral emission disappears because magnetic flux lines here leave Earth and hook up with the magnetic flux within the solar wind, which is that the constant flux of electrically charged particles ejected by the Sun. These are called open field lines and within the traditional picture, Jupiter and Saturn’s high-latitude polar regions aren’t expected to emit substantial auroras.
However, Jupiter’s X-ray auroras are inconsistent with this picture. They exist poleward of main auroral belt, pulsate regularly, and may sometimes vary at the North Pole from the South Pole . These are typical features of a ‘closed’ magnetic field , where the magnetic flux line exits the earth at one pole and reconnects with the earth at the opposite .
Using computer simulations, Zhonghua and colleagues previously found that the pulsating X-ray auroras might be linked to closed magnetic fields that are generated inside Jupiter then stretch out many kilometres into space before turning back.
On 16 and 17 July 2017, XMM-Newton observed Jupiter continuously for 26 hours and saw X-ray auroras pulsating every 27 min. Simultaneously, Juno had been travelling between 62-68 Jupiter radii above the planet’s pre-dawn areas. This was precisely the area that the team’s simulations suggested were important for triggering the pulsations. So, the team searched the Juno data for any magnetic processes that were occurring at an equivalent rate.
They found that the pulsating X-ray auroras are caused by fluctuations of Jupiter’s magnetic flux . because the planet rotates, it drags around its magnetic flux . The magnetic flux is struck directly by the particles of the solar radiation and compressed. These compressions heat particles that are trapped in Jupiter’s magnetic flux . This triggers a phenomenon called electromagnetic ion cyclotron (EMIC) waves, in which particles are directed along field lines.
The particles themselves are electrically charged atoms called ions. Guided by the sector , the ions ‘surf’ the EMIC wave across many KM of space, eventually slamming into the planet’s atmosphere and triggering X-ray aurora.
“What we see within the Juno data is that this beautiful chain of events. We see the compression happen, we see the EMIC wave triggered, we see the ions, then we see a pulse of ions traveling along field line. Then a couple of minutes later, XMM sees a burst of X-rays,” says William Dunn, Mullard Space Science lab , University College London, who co-led the research.
Now that process liable for Jupiter’s X-ray auroras has been identified for the 1st time, it exposes a wealth of possibilities for where it might be studied next. for instance , at Jupiter, the magnetic flux is filled with sulphur and oxygen ions that are spewed out by the volcanoes on the moon Io. At Saturn, the moon Enceladus jets water into space, filling Saturn’s magnetic flux with water ions.
“This may be a fundamental process that’s applicable to Saturn, Uranus, Neptune and probably’ exoplanets also ,” says Zhonghua.
It may be more widely applicable even than that because now that the method has been revealed, there’s a striking similarity to the ion auroras that happen here on Earth. within the case of Earth, the ion responsible may be a proton, which comes from a atom , and therefore the process isn’t energetic enough to create X-rays. Yet, basic process is that very same . So, Jupiter’s X-ray aurora is fundamentally an ion aurora, although at much higher energy than the proton aurora on Earth.
“It might be that EMIC waves play a crucial role in transferring energy from one place to a different across the cosmos,” says William.
As for Jupiter itself, the study of its auroras will continue with ESA’s Jupiter Icy moons Explorer (Juice). Set to arrive by 2029, Juice will study the planet’s atmosphere, magnetosphere, and therefore the effect that Jupiter’s 4 largest moons have on the auroras.
The findings were published in Science Advances.