Right: Sunlit view of the same hemisphere, with latitude and longitude superimposed...
The visible glow over Io's leading hemisphere produced by its atmosphere interacting with high-speed ions channeled by Jupiter's magnetic field. The more brightly glowing patches coincide with volcanic eruption plumes, where the atmosphere is densest. This image was recorded by Galileo when Io was in Jupiter's shadow in May 1998.
Sunlit view of the same hemisphere, with latitude and longitude superimposed (0 degrees longitude is at the centre of the side that always faces Jupiter, and 90 degrees longitude is the centre of the leading hemisphere).