Energetic neutral atoms from the heliosheath

We present the measurement and analysis of hydrogen energetic neutral atoms (ENAs) recorded with the NPD sensor of the ASPERA- 3 instrument on board Mars Express during the cruise phase and the Mars orbit phase. We conclude that the origin of these ENAs is the inner heliosheath. The ENA energy spectra are all very similar and can be fitted well by a two- component power law. The ENA intensities, integrated from 0.3 to 10 keV, vary in the range of 5; 103 to 3; 104 cm 2 sr 1 s 1. This report is an update of our earlier paper using the final NPD calibration data and improved sensor knowledge from two years of NPD operation. The present ENA measurements fit together well with earlier ENA data that were obtained from other spacecraft at higher energies, and which also have their likely origin in the inner heliosheath. Comparison of the measured ENA energy spectra with results from several heliospheric models shows that some of these models predict significantly lower ENA intensities at Earth orbit.