Electromagnetic ‘tug-of-war’ lights up Jupiter’s upper atmosphere

Researchers from the University of Leicester’s School of Physics and Astronomy, have for the first time, observed a complex ‘tug-of-war’ effect that lights up the aurora in Jupiter’s upper atmosphere, by using data from NASA’s Juno probe and the Hubble Space Telescope.

Researchers from the University of Leicester’s School of Physics and Astronomy have revealed evidence of a ‘tug-of-war’ that lights up aurorae in Jupiter’s upper atmosphere. The scientists employed data from Juno’s Magnetic Field Investigation (MAG), which measures Jupiter’s magnetic field from orbit around the gas giant, and observations from the Space Telescope Imaging Spectrograph carried by the Hubble Space Telescope.

The study, published in the Journal of Geophysical Research: Space Physics, describes the current delicate cycle driven by Jupiter’s rapid rotation and the release of sulphur and oxygen from volcanoes on its moon, Io.

The aurorae in Jupiter’s upper atmosphere

Their research provides the strongest evidence to date suggesting that Jupiter’s powerful aurorae is associated with an electric current system that acts as part of a ‘tug-of-war’ with material in the magnetosphere, and the region is dominated by the planet’s enormous magnetic field.

“We have had theories linking these electric currents and Jupiter’s powerful auroras for over two decades now, and it was so exciting to be able to finally test them by looking for this relationship in the data,” commented Dr Jonathan Nichols, Reader in Planetary Auroras at the University of Leicester and corresponding author for the study. “When we plotted one against the other, I nearly fell off my chair when I saw just how clear the connection is.

“It is thrilling to discover this relation because it not only helps us understand how Jupiter’s magnetic field works, but also those of planets orbiting other stars, for which we have previously used the same theories, and now with renewed confidence.”

Analysing lo

Despite its huge size, with a diameter more than 11 times that of Earth, Jupiter rotates once approximately every nine-and-a-half hours. Io is a similar size and mass to Earth’s moon, but orbits Jupiter at an average distance of 422,000 km: roughly 10% further away. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.

Scientists had long suspected a relationship between Jupiter’s aurorae and the material ejected from Io at a rate of hundreds of kilograms per second, but the data captured by Juno proved ambiguous.

“These exciting results on how Jupiter’s aurorae work are a testament to the power of combining Earth-based observations from Hubble with Juno measurements,” explained Dr Scott Bolton, of NASA’s Jet Propulsion Laboratory (JPL), and Principal Investigator (PI) for the Juno mission. “The HST images provide the broad overview, while Juno investigates close up.  Together they make a great team.”

Much of the material released from Io is propelled away from Jupiter by the planet’s rapidly rotating magnetic field, and as it moves outward its rotation rate tends to slow down. This results in an electromagnetic tug-of-war, in which Jupiter attempts to keep this material spinning at its rotation speed via a system of electric currents flowing through the planet’s upper atmosphere and magnetosphere.

Electric current

The component of the electric current flowing out of the planet’s atmosphere, carried by electrons fired downward along magnetic field lines into the upper atmosphere, was thought to drive Jupiter’s main auroral emission.

However, prior to Juno’s arrival this idea had never been tested, as no spacecraft with relevant instruments had previously orbited close enough to Jupiter. When Juno arrived in 2016, the expected signature of such an electric current system was not reported – and, while such signatures have since been found – one of the great surprises of Juno’s mission has been to the revelation that the nature of the electrons above Jupiter’s polar regions is much more complex than was initially expected.

The researchers compared the brightness of Jupiter’s main auroral emission with simultaneous measurements of the electric current flowing away from the Solar System’s largest planet in the magnetosphere over an early part of Juno’s mission.

These aurorae were observed with instruments on board the Hubble Space Telescope, in Earth orbit. By comparing the dawn-side measurements of its current with the brightness of Jupiter’s aurorae, the team demonstrated the relationship between the auroral intensity and magnetospheric current strength.

“Having more than five years of in-orbit data from the Juno spacecraft, together with auroral imaging data from the HST, we now have the material to hand to look in detail at the overall physics of Jupiter’s outer plasma environment, and more is to come from Juno’s extended mission, now in progress. We hope our present paper will be followed by many more exploring this treasure trove for new scientific understanding,” concluded Stan Cowley, Emeritus Professor of Solar-Planetary Physics at the University of Leicester and co-author for the study.

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