Direct Measurements of Energetic Neutral Hydrogen in the Interplanetary Medium

• Published in: The Astrophysical journal Vol. 644; no. 2; pp. 1317 - 1325
• Main Authors: Galli, A, Wurz, P, Barabash, S, Grigoriev, A, Lundin, R, Futaana, Y, Gunell, H, Holmström, M, Roelof, E. C, Curtis, C. C, Hsieh, K. C, Fedorov, A, Winningham, D, Frahm, R. A, Cerulli-Irelli, R, Bochsler, P, Krupp, N, Woch, J, Fraenz, M
• Format: Journal Article
• Language: English
• Published: IOP Publishing 20-06-2006
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/503765

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 x 10 super(3) and 10 super(5) cm super(-2) sr super(-1) s super(-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.