The Ionosphere and Plumes of Europa

We investigate the proposition that Europa emits a plasma plume, i.e a contiguous body of ionospheric plasma, extended in the direction of the coroation flow, analgous to the plume of smoke emitted in downwind direction from a smokestack. Such plumes were seen by Voyager 1 to be emitted by Titan. We find support for this proposition in published data from Galileo Plasma Science and Plasma Wave observations taken in the corotation wake of Europa and from magnetometer measurements reported from near the orbit, but away from Europa itself. This lends credence to the hypothesis that the plumes escaping from the ionosphere of Europa are wrapped around Jupiter by corotation, survive against dispersion for a fairly long time and are convected radially by magnetospheric motions. We present simple models of ionosphere aeronomy and plume acceleration and conclude that the plumes must consist of atomic oxygen ions flowing out, as observed, at subcorotational speed.