Direct Measurements of Energetic Neutral Hydrogen in the Interplanetary Medium

Abstract

We present an analysis of hydrogen energetic neutral atoms (ENAs) measured by the ASPERA-3 instrument on board Mars Express. We focus on ENAs that have no Martian origin. The energy spectra of these ENAs are all very similar and can be fitted well by a two-component power law. The fluxes, integrated from 0.2 to 10 keV, vary between 5 × 10³ and 10⁵ cm^-2 sr^-1 s^-1. We checked for possible sources for these ENAs, but we can rule out a planetary origin, a solar wind origin, contamination by UV from UV bright stars, and contamination by high-energy protons. With our present knowledge we conclude that the heliospheric termination shock is the most plausible source region.