Solar Storms Cause Enormous X-Ray Aurorae On Jupiter

Artist's impression of Jupiter's magnetosphere under the impact of the solar wind. JAXA

Everything is bigger on Jupiter, and that includes aurorae, also known as northern or southern lights. Now, a study of Jovinian aurorae coinciding with the arrival of a solar storm has, for the first time, witnessed X-ray aurorae that outshine those on Earth hundreds of times.

Aurorae result from interactions between the solar wind and a planet's magnetic field. Charged particles pushed out by the Sun disturb magnetospheric plasma. The shape of the magnetic field funnels these particles towards the north and south magnetic poles. When these charged particles encounter the upper atmosphere, they excite the atoms and molecules they encounter, leading to spectacular light shows.

Jupiter is five times Earth's distance from the Sun, so it experiences a weakened solar wind. On the other hand, its enormous magnetic field dwarfs that of the Earth. Even when the Sun is not particularly active, this can lead to impressive aurorae, which made astronomers wonder what would happen when a major solar storm sent its charged particles straight towards Jupiter.

"There's a constant power struggle between the solar wind and Jupiter's magnetosphere,” said William Dunn, a Ph.D. student at University College London, in a statement. “We want to understand this interaction and what effect it has on the planet. By studying how the aurora changes, we can discover more about the region of space controlled by Jupiter's magnetic field, and if or how this is influenced by the Sun. Understanding this relationship is important for the countless magnetic objects across the galaxy, including exoplanets, brown dwarfs and neutron stars."

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X-ray emissions viewed by the Chandra space telescope overlaid on Hubble telescope photographs of Jupiter during and after the arrival of a powerful coronal mass ejection. Joseph DePasquale, Smithsonian Astrophysical Observatory Chandra X-ray Center

In the Journal of Geophysical Research – Space Physics, Dunn described observations of a coronal mass ejection (CME) that hit Jupiter in October 2011. Despite the delay in analyzing what occurred, the paper is well timed. The Juno spacecraft will arrive at Jupiter in July, having been launched not long before the storms investigated in this paper occurred.

Among Juno's many goals is the study of Jupiter's magnetosphere, and Dunn's work will give its operators ideas on what to look for.

Dunn's study follows the discovery of X-ray emissions on Jupiter in the 1980s, followed by the identification in 2002 of a polar X-ray hotspot. The impact of the solar wind particles – hugely accelerated by Jupiter's magnetic field – on the atmosphere is so powerful, it causes the release of X-rays that are visible from the Chandra X-ray telescope in Earth orbit.

With the arrival of the CME, the X-rays became eight times as powerful as previous observations, and the hotspot's pulsing sped up from a period of 45 minutes to 26 minutes.

There has been debate as to whether the ions that cause the X-ray discharge come initially from the solar wind, or if they were previously part of the magnetosphere disturbed by an increase in wind strength. By measuring the dominant frequencies of the X-rays, Dunn found strong evidence of sulfur, which is common in Jupiter's atmosphere. However, he detected enough carbon to suggest some of the ions came from the solar wind, of which carbon is a major component.

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