page_title: Energetic neutral atom
section_title: Charge exchange
hierarchical_section_title: Energetic neutral atom / Creation of ENAs / Charge exchange
caption_attribution_description: English: In this image, a "hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA)".
context_page_description: Energetic neutral atom 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 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. These charged particles can be redirected by magnetic fields; for instance, Earth's magnetic field shields us from these particles. Every so often, a few of them steal electrons from neutral atoms they run into, making them neutral and not subject to large-scale electromagnetic fields. Still moving very fast, they tend to travel mostly in a straight line, subject to gravity. These are called Energetic Neutral Atoms. ENA images are constructed from the detection of these energetic neutral atoms.
  Earth's magnetosphere preserves Earth's atmosphere and protects us from cell-damaging radiation.
context_section_description: In a charge-exchange collision between a high energy plasma ion and a cold neutral atom, the ion 'gives' electrons by the neutral atom, producing a cold ion and an energetic neutral atom (ENA).
  I₁⁺ + A₂ → A₁ + I₂⁺
  where
  I₁⁺  plasma ion
  A₂  background neutral atom (lower energy)
  A₁  energetic neutral atom (ENA)
  I₂⁺  lower energy ion
  Species 1 and 2 may be the same or different and an exchange of two electrons is possible, e.g.
  H⁺ + H → H + H⁺
  
  Proton–hydrogen charge-exchange
  
  or
  He²⁺  + He  →   He  + He²⁺
  
  alpha-helium charge-exchange.
  Due to its charge neutrality, the resulting ENA is subject to gravitational forces only.  Because gravitation influences can normally be ignored, it is safe to assume that the ENA preserves the vector momentum of the original pre-interaction plasma ion.
  Some ENAs are lost in further charge-exchange, electron collisions and photoionization, but a great many travel very long distances in space completely undisturbed.
  Although plasma recombination and neutral atom acceleration by the solar gravitation may also contribute to an ENA population under certain conditions, the main exception to this creation scenario is the flux of interstellar gas, where neutral particles from the local interstellar medium penetrate the heliosphere with considerable velocity, which classifies them as ENAs as well.
caption: A hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA)[8]
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