What are energetic neutral atoms?
Energetic Neutral Atoms (ENAs) are particles with no charge that move relatively fast. ENAs are formed from particles that are ionized, meaning they have lost electronsA negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.. Sometimes, these ionsAn atom that has become electrically charged by the gain or loss of one or more electrons. interact with neutral atoms taking the electrons from those neutral atoms and becoming neutral themselves. Since the particle is no longer charged (it has equal numbers of protonsA massive, positively charged elementary particle; one of the main constituents of the nucleus of an atom. A proton is 1836 times heavier than an electron., and electrons) it no longer reacts to the magnetic fieldsA field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force., and travels in a straight line from the spot where the interaction occurred.
This interaction is called charge exchangeA process by which one atom gives up electrons to a proton or another atom. For example, if a positively-charged proton encounters a neutral atom, the neutral atom may give up one of its electrons to the proton. The proton-electron pair would then be neutral, and the formerly neutral atom would be positively-charged.. Charge exchange can happen between solar windA stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system. ions and neutral atoms from the Interstellar MediumAll the gas and dust found between stars.. Some of these ENAs happen to travel in just the right way so that they enter the IBEX spacecraft for collection. There are so many energetic particles that interact with interstellar neutrals, that even though they could travel in any direction, the IBEX sensors are able to pick up between 1 per hour and a few per minute.
About the Boundary of the Solar System