An aurora is the glow produced when charged particles from the Sun are funnelled by a planet's magnetic field into its upper atmosphere and collide with gas there. On Earth the colors follow the gas: oxygen gives green and deep red, nitrogen blues and purples. Activity tracks the solar cycle, and during severe geomagnetic storms the ovals around each pole expand far toward the equator. Aurorae are not unique to Earth; Jupiter, Saturn, Uranus, and Neptune all display them.