A collision often ends up transferring an electron from the hydrogen atom to the ring-current ion, without much change in the particles' energies. On the other hand, the ring current ion which has gained an electron is now neutralized, becoming a fast neutral atom with a great deal of energy. Since the Earth's magnetic field can only trap charged particles, the fast atom usually disappears quickly into distant space. Only the more energetic ones remain, since their probability of undergoing charge exchange is much lower. This process has an unusual application, allowing the ring current to observed remotely, a bit like the way astronomers observe distant stars through their telescopes.
Astronomers use light, which moves in straight lines.
Technically this is a rather difficult feat, because the number of ENAs coming from the ring current, especially outside magnetic storms, is rather small. A pilot experiment of this type ran for five weeks aboard the Swedish satellite Astrid, launched in December 1994, and produced some very simple ENA images. Substorms have now been studied for many years, from space and from the ground. Their details vary from one event to the next, just as thunderstorms in the atmosphere never seem alike, but many scientists have nevertheless concluded that they are a fundamental mode of energy release and of particle acceleration. Their main effect on the magnetosphere is the injection of many energetic ions and electrons from the tail, causing the ring current to grow significantly.