Probing the heliosphere with energetic hydrogen atoms

The idea of using energetic neutral atoms (ENAs), produced by charge exhange between energetic ions and ambient neutral atoms, as a diagnostic tool to investigate planetary magnetospheres from a distance has been extended to the investigation of the heliosphere. Here, we explore what one can reasonably expect of the heliospheric ENA (HSENA) and what criteria would be imposed on HSENA instruments by concentrating on 10-10³ keV protons in quiet-time interplanetary space, solar-flare events, corotating interaction regions, and anomalous cosmic rays. Simple modeling shows that energetic hydrogen atoms (EHAs) from these proton populations have distinctive signatures and that the detection of these particles can reveal energy, spatial, and temporal distributions of the protons in and out of the ecliptic, thus providing information on the acceleration and propagation of ions in three-dimensional interplanetary space, including the solar-wind termination shock. Such breadth of information could not be gained by in situ means.