Heliospheric Energetic Neutral Atoms: Numerical Modelling and Comparison with IBEX-Hi data

The Interstellar Boundary Explorer (IBEX) spacecraft has been measuring fluxes of the Energetic Neutral Atoms (ENAs) using the IBEX-Hi (0.3 – 6 keV) instrument since 2008. We have developed the numerical model to calculate ENA hydrogen fluxes employing reconstruction of the trajectories of the atoms (backward in time) from 1 au, where they are observed by IBEX, to the point of their origin in the inner heliosheath, i.e. the region of the perturbed solar wind between the termination shock and the heliopause, where the plasma is strongly heated (T ~ 10⁶ K). Along the trajectory of the atom, differential fluxes of the newly originated ENAs with a given speed are integrated, and losses due to ionization processes (charge exchange with protons and photoionization) are also taken into account.

The key factor in the simulation is a detailed consideration of the supra-thermal component of pickup ions (PUIs) that originate in the region of the supersonic solar wind and picked by the heliospheric magnetic field since this component is «parental» to the ENA. We take into account the supra-thermal component by solving the kinetic equation under the assumption that PUI distribution function is isotropic everywhere in the heliosphere. This method compares favorably with other existing approaches since it is based on fundamental physical laws.

We have calculated model maps of the ENA fluxes based on the previously developed kinetic-MHD models of the SW/LISM interaction (Izmodenov & Alexashov, 2015, 2020), and performed the comparison with IBEX-Hi data. The IBEX-Hi data is one of the few sources of knowledge about the structure of the heliospheric boundary, imposing significant limitations on the parameters of the model of the heliosphere. As a result of the comparison, we concluded that 1) ENA fluxes from the region of the inner heliosheath are extremely sensitive to the form of PUI distribution function; 2) the model of the heliosphere Izmodenov & Alexashov (2020) that differs from the model Izmodenov & Alexashov (2015) in configuration of the interstellar magnetic field reproduces the IBEX-Hi data better; 3) despite a relatively good agreement, there are some qualitative differences between the model calculations and IBEX-Hi data in some energy channels of IBEX-Hi. The reasons for these differences are discussed.