Jupiter's visible aurora and Io footprint

Images obtained by the Galileo spacecraft's solid‐state imaging (SSI) system represent the first survey of Jupiter's northern auroral emissions at visible wavelengths and on the nightside of the planet. These images captured the emissions with unprecedented spatial resolutions down to ∼26 km pixel −1 . Four classes of emission were observed: (1) a continuous, primary arc associated with the middle/outer magnetosphere, (2) a variable secondary arc associated with the region just beyond Io's torus, (3) diffuse “polar cap” emission, and (4) a patch and tail associated with the magnetic footprint of Io. The primary arc emission occurs at an altitude 245±30 km above the 1‐bar pressure level. Its horizontal width is typically a few hundred kilometers, and its total optical power output varied between ∼10 10 and ∼10 11 W in observations taken months apart. The location of the primary arc in planetary coordinates is similar to that on dayside images at other wavelengths and does not vary with local time. The morphology of the primary arc is not constant, changing from a multiply branched, latitudinally distributed pattern after dusk to a single, narrow arc before dawn. Emission from Io's ionospheric footprint is distinct from both the primary and secondary arcs. Measurements of its optical power output ranged from 2 to 7×10 8 W.