Simulating the compton-getting effect for hydrogen flux measurements: Implications for IBEX-Hi and -Lo observations

The Interstellar Boundary EXplorer (IBEX), launched in 2008 October, has improved our understanding of the solar wind-local interstellar medium interaction through its detection of neutral atoms, particularly that of hydrogen (H). IBEX is able to create full maps of the sky in six-month intervals as the Earth orbits the Sun, detecting H with energies between ∼0.01 and 6 keV. Due to the relative motion of IBEX to the solar inertial frame, measurements made in the spacecraft frame introduce a Compton-Getting (CG) effect, complicating measurements at the lowest energies. In this paper we provide results from a numerical simulation that calculates fluxes of H atoms at 1 AU in the inertial and spacecraft frames (both ram and anti-ram), at energies relevant to IBEX-Hi and -Lo. We show theory behind the numerical simulations, applying a simple frame transformation to derived flux equations that provides a straightforward way to simulate fluxes in the spacecraft frame. We then show results of H energetic neutral atom fluxes simulated at IBEX-Hi energy passbands 2-6 in all frames, comparing with IBEX-Hi data along selected directions, and also show results simulated at energies relevant to IBEX-Lo. Although simulations at IBEX-Hi energies agree reasonably well with the CG correction method used for IBEX-Hi data, we demonstrate the importance of properly modeling low energy H fluxes due to inherent complexities involved with measurements made in moving frames, as well as dynamic radiation pressure effects close to the Sun.