An Energetic Neutral Atom (ENA) is a neutral atom, typically Hydrogen, Helium, Oxygen, or Sulfur, moving rapidly. Typical creation in space is at the border between two clouds, where neutral atoms collide with ions. Energy can be transferred to neutral atoms or energetic ions can gain an electron in the collision.
ENAs are little affected by outside forces and can travel long distances. Thus they can be used to image Solar System phenomena such as the border between the ISM and the Heliosphere. They can also show magnetic phenomena.
Interstellar Boundary Explorer (IBEX)