... 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. ...