A team from the Laboratory of Atmospheric and Planetary Physics (LPAP) at the University of Liege has used NASA's Juno spacecraft to resolve, for the first time, fine-scale features in Ganymede's auroras, which develop in the moon's thin oxygen atmosphere under the influence of its intrinsic magnetic field, the only such field known around a moon in the solar system.
Auroras have fascinated observers for centuries as diffuse, shifting curtains of red, green, purple and blue light that typically appear at high latitudes on Earth, although strong solar activity, such as the recent peak of the 11 year solar cycle, can drive them to mid latitudes where they become visible over much broader regions.
On Earth, these displays occur when the solar wind interacts with the planet's magnetic field, driving powerful electromagnetic processes that channel charged particles into the upper atmosphere, where they collide with gases such as oxygen and nitrogen and cause them to emit visible light.
Auroras are not unique to Earth and have been detected at other solar system bodies including Venus, Mars, Jupiter, Saturn and Uranus, but until now limitations in spatial resolution left key questions open about how closely auroras at different worlds resemble one another on small scales.
Juno, which has been orbiting Jupiter since 2016 and is approaching a decade of operations, provided a unique opportunity to investigate Ganymede's auroras during a high speed flyby on 7 July 2021 that brought the spacecraft close to the icy moon.
Philippe Gusbin, whose master's thesis in Space Sciences underpins the new analysis, notes that earlier observations of Ganymede's auroras, particularly from ground based facilities, could not resolve the small scale structures that are typical of planetary auroral emissions and thus offered only a coarse view of the phenomenon.
Using Juno's ultraviolet spectrograph (UVS), the LPAP team achieved spatial resolutions of a few kilometres, allowing them to reconstruct the overall shape of Ganymede's ultraviolet aurora from a set of narrow strips acquired as the spacecraft swept quickly past the moon.
When the researchers examined these high resolution strips in detail, they found that the auroral emission is not a uniform curtain but instead breaks up into a chain of discrete patches, revealing a fragmented structure across the auroral region.
Similar patchy forms, often described as "beads", have previously been documented in Earth's and Jupiter's auroras, where they are associated with sub storms and dawn storms, events that involve large scale rearrangements of a magnetosphere and the rapid release of stored energy that intensifies auroral activity.
Ganymede interacts with Jupiter's magnetospheric plasma in a way that the authors compare to Earth's interaction with the solar wind, and the detection of Ganymede auroral patches resembling terrestrial structures points to common underlying mechanisms that can arise whenever a magnetised body couples to its surrounding plasma environment and external drivers.
The new results appear in the journal Astronomy and Astrophysics in a letter titled "Juno's high spatial resolution ultraviolet observations of Ganymede's auroral patches. Constraints on the magnetospheric source region," which uses the morphology of the patches to place limits on where in Ganymede's magnetosphere the responsible processes occur.
Although Juno's close look at Ganymede lasted less than 15 minutes and the spacecraft will not repeat such a flyby, leaving open questions about how frequently the patches occur and how they evolve, future missions are expected to build on these findings with longer term monitoring.
The European Space Agency's Juice (Jupiter Icy Moons Explorer) mission, now en route to Jupiter with arrival planned for 2031, carries an ultraviolet spectrograph similar to Juno's and will dedicate extensive observation time to Ganymede's auroras, offering the prospect of tracing their variability and uncovering additional details of the moon's space environment.
Research Report:Juno's high spatial resolution ultraviolet observations of Ganymede's auroral patches. Constraints on the magnetospheric source region
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