Detection of 55–80 keV Hydrogen Atoms of Heliospheric Origin by CELIAS/HSTOF on<i>SOHO</i>

The High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 is capable of identifying energetic hydrogen atoms (EHAs) between 55 and 80 keV. Between 1996 February 13 and 1997 August 31, near solar minimum, there were 285 "quiet" days when the interplanetary charged-particle flux was low. During these quiet times, HSTOF scanned the apex of the heliosphere once and the antiapex twice. The flux level and time profile, and hence the arrival direction, of the EHAs accumulated during these quiet times are best interpreted as fluxes of EHAs coming from the heliosheath.