Scientists from the United States, Europe, and China used the Ultraviolet Spectrometer (UVS) aboard NASA’s Juno spacecraft to map the detailed patch structure of Ganymede’s aurora, which is comparable to those seen on Earth. Their findings show that interactions between magnetic fields and charged particles may be the universal engine of auroras, with implications for our understanding of the magnetosphere throughout the solar system.
Artist’s concept of the aurora borealis on Jupiter’s moon Ganymede. Image credits: NASA/ESA/G. Bacon, STScI/J. Saur, University of Cologne.
Ganymede is the only moon known to have its own intrinsic magnetic field, maintaining a miniature magnetosphere embedded within Jupiter’s magnetosphere.
Its aurora mainly originates from oxygen emission at wavelengths of 130.4 nm and 135.6 nm, excited by precipitating electrons.
In a new study, University of Liège researcher Philippe Gusbin and his colleagues analyzed ultraviolet observations of Ganymede recorded on June 7, 2021 by the Juno spacecraft.
They confirmed multiple auroral spots in the leading downstream hemisphere of the moon.
The typical size of the patch is about 50 km, and the brightness reaches about 200 Rayleighs.
“Auroras have also been observed at Ganymede and are caused by the precipitation of electrons in its thin oxygen atmosphere,” Gusvin explained.
“Observations of Ganymede’s auroras before Juno were limited by the spatial resolution of ground-based observations and were unable to resolve the small-scale structures typical of planetary auroras.”
The form and scale of this feature is similar to the auroral “beads” seen on Earth before magnetospheric substorms and on Jupiter during so-called dawn storms.
The apparent absence of a similar patch in the southern hemisphere may be due to observational geometry, but an asymmetry related to Ganymede’s position within Jupiter’s plasma disk cannot be ruled out.
“‘Beads’ have been observed in the auroras of Earth and Jupiter, where they are associated with substorms and dawn storms, large-scale rearrangements of the magnetosphere that release huge amounts of energy and create intense auroral activity,” said Dr. Alessandro Moirano, a postdoctoral researcher at the University of Liège and the National Institute of Astrophysics in Rome.
This finding suggests that comparable physical mechanisms may operate throughout the magnetosphere, despite large differences in scale and environment.
“Juno’s close observation of Ganymede lasted less than 15 minutes, and the spacecraft will never fly over Ganymede again, so we don’t know how common these spots are or how they will change over time,” said Dr. Bertrand Bonfont, an astrophysicist at the University of Liege.
“Fortunately, ESA’s JUICE mission is currently on its way to Jupiter and is scheduled to arrive in 2031 to carry out specialized observations of Ganymede.”
“This spacecraft is equipped with an ultraviolet spectrometer similar to Juno. This will allow us to collect observations over long periods of time, monitor the evolution of Ganymede’s aurora, and hopefully uncover new mysteries.”
a paper The discovery was published in a magazine astronomy and astrophysics.
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A. Moirano others. 2026. High spatial resolution ultraviolet observations of Ganymede’s aurora patches by Juno. Constraints on the magnetospheric source region. A&A 706, L16; doi: 10.1051/0004-6361/202558379