Helium Energetic Neutral Atoms - a New Perspective for Heliospheric and Extraheliospheric Observations with IMAP

Abstract:

Helium as the second most abundant species in the solar wind as well as in the interstellar medium should be prominent in the observations made be ENA detectors. Since IBEX-Hi detector was not equipped with a mass spectrometer, He ENA contribute only negligibly to the overall signal observed with the detector and are indistinguishable from the hydrogen ENAs. The situation will likely change with the ENA detector on IMAP. In our work we assess the expected heliospheric and potential extraheliospheric emission of He ENAs and show potential ability of He ENAs to resolve the structure of the LISM in the proximity of the heliosphere.

We assess the heliospheric emission using a simple model of the heliosphere that takes the Voyager observations into account. We assume helium ion spectra at the termination shock and propagate them through the inner heliosheath. The computed distributions are then used to integrate the He ENA fluxes. To assess the IBEX Ribbon emission we adapt the analytical model of the Secondary ENA emission by Moebius et al. 2013 for helium. We obtain that both the inner heliosheath and the Ribbon emissions are much weaker than the observed H ENA by IBEX, except from the heliotail.

One of the possible explanations for the IBEX Ribbon proposed by Grzedzielski et al. 2010 suggests that the signal originates in the boundary region between the LIC and the cavity of the Local Bubble. The main disadvantage of the model is the necessity of a short distance to this interface and low plasma density in the LIC, so that ionization processes do not extinct the signal. However, the mean free path for He ENAs ionization could be longer by an order of magnitude and reaches about 8000 AU. This should allow us to observe ENAs originating from suprathermal ions created in processes likely operating at the LIC boundaries, the distance to which could be ~0.1 pc. This makes He ENA observations a unique tool to observe such regions, currently inaccessible to optical telescopes.