The Downwind Hemisphere of the Heliosphere: Eight Years of IBEX-Lo Observations

We present a comprehensive study of energetic neutral atoms (ENAs) of 10 eV to 2.5 keV from the downwind hemisphere of the heliosphere. These ENAs are believed to originate mostly from pickup protons and solar-wind protons in the inner heliosheath. This study includes all low-energy observations made with the Interstellar Boundary Explorer over the first eight years. Because the protons around 0.1 keV dominate the plasma pressure within the inner heliosheath in downwind direction, these ENA observations offer the unique opportunity to constrain the plasma properties and dimensions of the heliosheath where no in situ observations are available. We first derive energy spectra of ENA intensities averaged over time for 49 macropixels covering the entire downwind hemisphere. The results confirm previous studies regarding integral intensities and the roll-over around 0.1 keV energy. With the expanded data set, we now find that ENA intensities at 0.2 and 0.1 keV seem to anti-correlate with solar activity. We then derive the product of total plasma pressure and emission thickness of protons in the heliosheath to estimate lower limits on the thickness of the inner heliosheath. The temporally averaged ENA intensities support a rather spherical shape of the termination shock and a heliosheath thickness between 150 and 210 au for most regions of the downwind hemisphere. Around the nominal downwind direction of 76° ecliptic longitude, the heliosheath is at least 280 au thick. There, the neutral hydrogen density seems to be depleted compared to upwind directions by roughly a factor of 2.