![\"Juno](\"https:\/\/cdn.sci.news\/images\/enlarge13\/image_14189e-Callisto-Footprint.jpg\")
<\/a><\/p>\nJuno capturing the marks on Jupiter of all four Galilean moons; the aurorae related to each are labeled Io, Eur (for Europa), Gan (for Ganymede), and Cal (for Callisto). Image credit: NASA \/ JPL-Caltech \/ SwRI \/ UVS Team \/ MSSS \/ Gill \/ J\u00f3nsson \/ Perry \/ Hue \/ Rabia.<\/p>\n
Before NASA\u2019s Juno mission, three of Jupiter\u2019s four largest moons, known as Galilean moons \u2014 Io, Europa, and Ganymede \u2014 were shown to produce these distinct auroral signatures.<\/p>\n
But Callisto, the most distant of the Galilean moons, remained a mystery.<\/p>\n
Despite multiple attempts using the NASA\/ESA Hubble Space Telescope, Callisto\u2019s footprint had proven elusive, both because it is faint and because it most often lies atop the brighter main auroral oval, the region where aurorae are displayed.<\/p>\n
NASA\u2019s Juno mission, orbiting Jupiter since 2016, offers unprecedented close-up views of these polar light shows.<\/p>\n
But to image Callisto\u2019s footprint, the main auroral oval needs to move aside while the polar region is being imaged.<\/p>\n
And to bring to bear Juno\u2019s arsenal of instruments studying fields and particles, the spacecraft\u2019s trajectory must carry it across the magnetic field line linking Callisto and Jupiter.<\/p>\n
These two events serendipitously occurred during Juno\u2019s 22nd orbit of the giant planet, in September 2019, revealing Callisto\u2019s auroral footprint and providing a sample of the particle population, electromagnetic waves, and magnetic fields associated with the interaction.<\/p>\n
Jupiter\u2019s magnetic field extends far beyond its major moons, carving out a vast region (magnetosphere) enveloped by, and buffeted by, the solar wind streaming from our Sun.<\/p>\n
Just as solar storms on Earth push the northern lights to more southern latitudes, Jupiter\u2019s aurorae are also affected by our Sun\u2019s activity.<\/p>\n
In September 2019, a massive, high-density solar stream buffeted Jupiter\u2019s magnetosphere, briefly revealing \u2014 as the auroral oval moved toward Jupiter\u2019s equator \u2014 a faint but distinct signature associated with Callisto.<\/p>\n
This discovery finally confirms that all four Galilean moons leave their mark on Jupiter\u2019s atmosphere, and that Callisto\u2019s footprints are sustained much like those of its siblings, completing the family portrait of the Galilean moon auroral signatures.<\/p>\n
\u201cOur observations confirm the electrodynamic coupling between Callisto and Jupiter,\u201d said Dr. Jonas Rabia, a researcher at the Institut de Recherche en Astrophysique et Plan\u00e9tologie and CNRS, and colleagues.<\/p>\n
\u201cThis coupling will be further analyzed by NASA\u2019s JUICE mission, successfully launched in April 2023, which will repeatedly visit Callisto and its local environment, enabling a better characterization of the interaction of Callisto with Jupiter\u2019s magnetosphere.\u201d<\/p>\n
\u201cThe reported in situ and remote observations complete the family portrait of Galilean moon auroral footprints, and resolve the longstanding mystery as to whether Callisto\u2019s electromagnetic interaction is fundamentally different from the inner three Galilean satellites.\u201d<\/p>\n
\u201cThe observed similarities, both in the auroral structure and the electron in situ properties, point towards a universal physical mechanism at work for moon-planet and planet-star magnetospheric interactions, relevant for other binary systems that are only accessible remotely, in the Solar System and beyond.\u201d<\/p>\n