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\nJuno capturing the marks on Jupiter of all four Galilean moons. The auroras related to each are labeled Io, Eur (for Europa), Gan (for Ganymede), and Cal (for Callisto).
\nNASA\/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 (Io, Europa, and Ganymede) were shown to produce these distinct auroral signatures. But Callisto, the most distant of the Galilean moons, remained a mystery. Despite multiple attempts using NASA\u2019s 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 auroras are displayed.<\/p>\n
NASA\u2019s Juno mission, orbiting Jupiter since 2016, offers unprecedented close-up views of these polar light shows. But to image Callisto\u2019s footprint, the main auroral oval needs to move aside while the polar region is being imaged. And to bring to bear Juno\u2019s arsenal of instruments studying fields and particles, the spacecraft’s 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
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