In charge exchange reactions, illustrated schematically in the figure below, a singly charged ion (such as H+ or O+) collides with a neutral atom or molecule and captures one of its electrons, thereby becoming a neutral atom (panel a). Only a small amount (a few eV) of energy is transferred to the electron donor, so the newly created neutral retains most of the original energy (a few to hundreds of keV) of the ion. Since it no longer carries a charge, the energetic neutral atom (ENA) is not affected by electric and magnetic fields (see the entry on plasma) and travels along a straight, line-of-sight path from the point where it was created (panel b). An imager on a spacecraft several hundreds or several thousands of kilometers from a particular ENA source region (such as the Earth's ring current) can detect the ENA flux from that region, just as an optical telescope detects photons emitted from a distant astronomical object. The counts from the neutral atom detector can then be used to construct images of the ENA emission region. The number of ENA emitted from a given region of space depends on the energy and species of the energetic ion population in that region and on the density of the neutral gas with which the ions undergo charge exchange. In the case of the Earth's inner magnetosphere, most of the ENAs are produced through charge exchange with geocoronal neutral hydrogen, which is the primary process by which particles are lost from the ring current.