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Can IBEX Detect Interstellar Neutral Helium Or Oxygen From Anti-ram Directions?

by Galli et al.

Abstract:To better constrain the parameters of the interstellar neutral flow we searched the IBEX-Lo database for helium and oxygen from the interstellar medium in the anti-ram direction in the three years (2009–2011) with the lowest background rates. We found that IBEX-Lo cannot observe interstellar helium from the antiram direction because the helium energy is too low for indirect detection by sputtering off the IBEX-Lo conversion surface. Our results show that this sputtering process has a low energy threshold between 25 and 30 eV, whereas the energy of the incident helium is only 10 eV for these observations. Interstellar oxygen, on the other hand, could in principle be detected in the anti-ram hemisphere, but the expected magnitude of the signal is close to the detection limit imposed by counting statistics and by the magnetospheric foreground.

Fig. 9 of Galli et al. 2015: The steep decrease of the measured ISN helium signal around spin angle 180° implies, by comparison with the model results, that the low energy cut-off to detect neutral helium with IBEX-Lo is situated around 25-30 eV.