Global Parameter Estimation of Interstellar Helium from IBEX-Lo observation

The Interstellar Boundary Explorer carries the IBEX-Lo instrument that measures inflowing interstellar neutral atoms (H, He, Ne, O) and has made fundamental advancements in our understanding of local interstellar flow parameters. The fixed orientation of IBEX-Lo viewing, roughly perpendicular to the radial direction of the Sun, leads to a short observation interval along the orbit resulting in a degeneracy in the determination of the interstellar flow parameters. This degeneracy is expressed in terms of the interstellar neutral (ISN) flow speed, temperature, and inflow latitude (as a function of inflow longitude), referred to as the IBEX 4D parameter tube. Observations of the maximum flux longitude at 1 au (are key to defining the parameter tube orientation. Several studies have attempted to break the parameter tube degeneracy, although an absolute measurement of these parameters is hindered by the presence of H, of He secondary populations, and other species (neon and oxygen).

In this study, we estimate the parameters of Interstellar Helium by comparing the observed rates with rates obtained from the analytical Full Integration Model (aFINM) developed at UNH. We use a methodology similar to that used by Schwadron et al. (2016) applied to derive H and He flow parameters for their primary and secondary components. We forward model the complete IBEX-Lo response function in the spacecraft frame of reference and numerically integrate to obtain simulated rates. This method differs from adopting closed-form solutions in the Earth frame of reference, which was developed by Lee et al. (2012) and implemented by Möbius et al. (2012). This is the first study where we analyze the aFNIM with simultaneous effect of the primary and secondary populations for chi-square minimization without assumptions about primary or secondary population parameters.