Here is a top-down illustration of the satellite’s sunlit leading & trailing hemispheres orbiting in the Io-plasma torus with the three external influences highlighted. Red vectors = the O2 source (sputtering). Yellow vectors = the solar radiation, fixed at the bottom of the page. Blue vector = the revolution around Jupiter. The above geometry of the three external influences results in a 'Rotation-Driven exosphere." The molecular oxygen components of Europa and Ganymede's exospheres are thought to be rotation-driven. On these two satellites, the result of this rotation is a large atmospheric bulge near dusk, local time.