DETECTING NEUTRAL ATOMS FROM BEYOND THE HELIOPAUSE WITH INTERSTELLAR BOUNDARY EXPLORER

Theoretical work concerning neutral atoms to be observed by the recently launched Interstellar Boundary Explorer ( IBEX ) spacecraft has focused on neutral hydrogen atoms created by charge exchange in the inner heliosheath. In this Letter, we explore the effects of including H atoms born through charge exchange in the outer heliosheath and the nearby interstellar medium, with energies between 10 eV and ∼70 eV. Although such atoms are generally not regarded as energetic neutral atoms (ENAs), they can nevertheless be observed by IBEX due to the spacecraft's orbital speed around the Sun. We find that the apparent boost in particle energy due to this motion allows IBEX to observe interstellar neutral atoms, and that the flux of atoms at the lowest energies observed by IBEX will be dominated by atoms originating from outside the heliopause. We show, using simulated data from a self-consistent model of the heliosphere, that the first two energy channels on IBEX will be dominated by outer heliosheath ENAs, and that these flux maps can, in principle, be used to deduce the asymmetry of the outer heliosphere caused by the interstellar magnetic field. The work presented in this Letter employs the simplifying assumption that radiation pressure exactly balances gravity for an H atom, and the predictions made here are only appropriate for solar minimum conditions.