The "ring of fire" seen by satellite cameras around the polar caps (picture below) is probably created by electrons leaking from the tail's plasma sheet, whose magnetic field lines form a somewhat imperfect trap. ... One possible explanation involves the close association between auroral arcs and electric currents which flow along field lines between the ionosphere and distant space ( Birkeland currents ). ... That produces an extra resistance to the flow of current. ...
... A typical density of energetic ions is 1 per cubic cm., and the ions are matched by electrons, generally of lower energy. ... This region is almost empty of plasma--typical density 0.01 ion/cubic cm., the "best vacuum" in the Earth's vicinity--but it contains a relatively strong magnetic field which, since it fills a large volume, can store appreciable magnetic energy, Many believe that this is the storehouse from which substorms draw their energy, releasing it quite rapidly. ...
Ice Volcanoes Reshape Europa's Chaotic Surface . Ice-spewing volcanoes and the grinding and tearing of tectonic plates have reshaped the chaotic surface of Jupiter's frozen moon Europa, images from NASA's Galileo spacecraft reveal. ... Although the images do not show currently active ice volcanoes or geysers, they do reveal flows of material on the surface that probably originated from them, said Galileo imaging team member Dr. Ronald Greeley of Arizona State University, Tempe. ... Europa . ...