Data from Juno and Hubble Reveal that an Electromagnetic ‘tug-of-war’ Lights Up Jupiter’s Upper Atmosphere

Using data from NASA’s Juno probe and the Hubble Space Telescope, new Leicester space research has revealed, for the first time, a complex ‘tug-of-war’ lights up aurorae in Jupiter’s upper atmosphere.

The study, published in the Journal of Geophysical Research: Space Physics, reveals the delicate current cycle caused by Jupiter’s rapid rotation and the emission of sulphur and oxygen from Io’s volcanoes.

Researchers from the University of Leicester’s School of Physics and Astronomy used 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.

Their findings provide the most compelling evidence yet that Jupiter’s intense aurorae are linked to an electric current system that engages in a ‘tug-of-war’ with material in the magnetosphere, the zone dominated by the planet’s massive magnetic field.

Dr. Jonathan Nichols is a Reader in Planetary Auroras at the University of Leicester and corresponding author for the study. He said:

“We’ve 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. And when we plotted one against the other I nearly fell off my chair when I saw just how clear the connection is.”

“It’s 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.”

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.

Professor Stan Cowley

Jupiter rotates once every nine and a half hours, despite having a diameter more than 11 times that of Earth.

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 hypothesized a link between Jupiter’s aurorae and the debris blasted from Io at hundreds of kilos per second, but Juno’s findings proved ambiguous.

Dr. Scott Bolton, of NASA’s Jet Propulsion Laboratory (JPL), is Principal Investigator (PI) for the Juno mission. He said:

“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. The HST images provide the broad overview, while Juno investigates close up. Together they make a great team!”

Much of the material ejected from Io is driven out from Jupiter by the planet’s rapidly revolving magnetic field, which slows down as it goes outward. This causes an electromagnetic tug-of-war in which Jupiter strives to maintain this material spinning at its rotation speed using an electric current system that flows through the planet’s upper atmosphere and magnetosphere.

Jupiter’s major auroral emission was considered to be driven by 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.

However, before to Juno’s arrival, no spacecraft with suitable equipment had orbited close enough to Jupiter to put this theory to the test. When Juno arrived in 2016, the expected signature of such an electric current system was not reported, and while such signatures have since been discovered, one of the great surprises of Juno’s mission has been to show that the nature of the electrons above Jupiter’s polar regions is much more complex than was previously thought.

Over an early period of Juno’s mission, the researchers contrasted the brightness of Jupiter’s major auroral emission with simultaneous measurements of the electric current flowing away from the Solar System’s largest planet in the magnetosphere.

These auroras were observed using instruments on the Hubble Space Telescope, which is in orbit around the Earth. The researchers proved the association between auroral intensity and magnetospheric current strength by comparing dawn-side current measurements with the brightness of Jupiter’s aurorae.

Stan Cowley is Emeritus Professor of Solar-Planetary Physics at the University of Leicester and co-author for the study, and has studied Jupiter’s powerful aurorae for 25 years. Professor Cowley added:

“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.”