Juno probe provides evidence of magnetic reconnection between Jupiter and Ganymede
In June 2021, NASA's Juno spacecraft approached Ganymede, Jupiter's largest moon, and flew past its magnetosphere. Observational data showed signs of magnetic reconnection – a process by which magnetic field lines are able to merge and reorient.
Ganymede is Jupiter 's largest natural satellite and the largest in the entire Solar System. It is also the only satellite in our system to have its own magnetic field. It probably results from convection movements in the ferrous core of this object. This magnetosphere, however, fits into Jupiter's magnetic field, which is much stronger. Both are connected via field lines. " The breaking and reconnecting of Ganymede's magnetic field lines with Jupiter's creates magnetospheric fireworks ," said Scott Bolton . He is a principal investigator of the Juno mission, at the Southwest Research Institute (SwRI).
Accelerated and aligned electron flows
Magnetic reconnection is an explosive physical process. It converts stored magnetic energy into kinetic energy and heat. This causes surrounding charged particles to accelerate and heat up. Such magnetic reconnection is expected to occur when Ganymede's mini-magnetosphere interacts with Jupiter's massive magnetosphere, in an area called the magnetopause. It is the border between the two regions.
Bolton and his collaborators used the data relayed by Juno. For this, they used the JADE experiment ( Jovian Auroral Distributions Experiment ) on board. They were thus able to examine the behavior of particles (electrons and ions) and magnetic fields, when the field lines of Jupiter and Ganymede merge, break up and reorient themselves. Researchers can therefore potentially identify physical evidence of the magnetic reconnection phenomenon.
Interpretation of magnetic topology and electron flow direction for two different reconnection scenarios at the magnetopause upstream of Ganymede. The yellow dotted line indicates Juno's trajectory. Credits: Ebert et al., Geophysical Research Letters (2022)
They report observing increased electron flows. This is particularly true for accelerated electrons (up to 2-3 keV/q) and aligned on the magnetic field at Ganymede's magnetopause. Scientists take this as evidence that a magnetic reconnection occurred as the Juno spacecraft passed. " These observations support the idea that magnetic reconnection at Ganymede's magnetopause may be a driver of dynamical processes in the local space environment around this moon ," says Robert Ebert. He is the lead author of the article describing the results .
A reconnection between Jupiter and Ganymede linked to the aurora
Before Juno, the Galileo probe, launched by NASA in 1989, is the only one to have placed itself in orbit around Jupiter. She stayed there for eight years (from 1995 to 2003). The first of six close flybys of Ganymede, carried out in 1996, led to the discovery of the magnetic field of this satellite. “ Galileo's observations revealed abrupt rotations in the orientation of the magnetic field. He also observed variations in the amplitude of the magnetic field and abrupt changes in the energy properties of particles, plasma waves and thermal plasma [at the magnetopause] ”, relate the researchers. These features suggested magnetic reconnection.
The Juno probe was launched in 2011. It has been in orbit around Jupiter since July 2016. On June 7, 2021, it flew close to Ganymede (up to 1046 km from the surface). This flyby included a clear crossing of the magnetopause upstream of Ganymede on the outgoing segment of its transit. This is where evidence of magnetic reconnection was observed. " The countercurrent electrons observed on both sides of the magnetopause suggest the presence of multiple reconnection sites, both north and south of the spacecraft ," the researchers said.
Scientists believe that this reconnection is linked to the generation of auroras observed around the poles of the satellite. " The accelerated electrons that JADE observed are similar to those observed by NASA's MSS (Magnetospheric Multiscale) spacecraft during reconnection to Earth's magnetopause ," says Dr. Stephen Fuselier. He is a researcher at SwRI and co-author of the article.
A taste of future missions to Jupiter
Thus, JADE has highlighted the universal aspect of the phenomenon of magnetic reconnection. Despite the great differences that exist between Ganymede and our planet, they have common points related to this physical process. “ This is the first measurement of this complicated interaction on Ganymede. This gives us a very tantalizing taste of the insights we expect from ESA's JUICE mission ,” said Thomas Greathouse, Juno Mission Scientist at SwRI.
The JUICE ( JUpiter ICy moons Explorer ) mission, scheduled for launch in April 2023, is the European Space Agency's next big mission. It will be the first time that an exclusively European probe will travel to the outer solar system. As its name suggests, this mission is dedicated to observing Jupiter's moons, including Ganymede, Europa and Callisto. Arrival in the Jovian system expected in July 2031.
The SwRI has built two other ultraviolet spectrographs (similar to the one on board Juno). They will be used to further examine the magnetosphere of Jupiter and Ganymede. One will be transported by the JUICE probe, the other will be on board Europa Clipper. It is a probe that NASA has developed. NASA should launch it in October 2024, to arrive on Jupiter in 2030.
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