Energetic neutral atom Energetic neutral atom Energetic neutral atom (ENA) imaging, often described as "seeing with atoms", is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere, even to its outer boundary. This constitutes the far-flung edge of the solar system. The solar wind consists of ripped-apart atoms (called plasma) flying out of the Sun. This is mostly hydrogen, that is, bare electrons and protons, with a little bit of other kinds of nuclei, mostly helium. The space between solar systems is similar, but they come from other stars in our galaxy.
Degree of ionization plasma is in a "Coulomb-collision dominated regime", i.e. when formula_9, which can correspond to a degree of ionization as low as 0.01%. A "partially" or "weakly ionized gas" means the plasma is not dominated by Coulomb collisions, i.e. when formula_10. A particular case of fully ionizes gases are very hot thermonuclear plasmas, such as plasmas artificially produced in nuclear explosions or naturally formed in our Sun and all stars in the universe. Stars contain largely hydrogen and helium gases, that are "fully ionized" into electrons, protons (H) and helium ions (He). Equations to characterize such very hot, fully ionized thermonuclear