Heliospheric Energetic Neutral Atoms: Non-stationary Modeling and Comparison with IBEX-Hi data

TL;DR

This paper presents a non-stationary numerical model of heliospheric ENAs that successfully reproduces IBEX-Hi observations, highlighting the importance of PUI velocity distributions and identifying areas for quantitative improvement.

Contribution

The study introduces a time-dependent kinetic model of heliospheric ENAs that incorporates detailed PUI distributions and compares favorably with IBEX-Hi data.

Findings

Model reproduces IBEX-Hi GDF multi-lobe structure

GDF highly sensitive to PUI velocity distribution

Quantitative differences suggest further model refinement

Abstract

The Interstellar Boundary Explorer (IBEX) has been measuring fluxes of the Energetic Neutral Atoms (ENAs) using the IBEX-Hi (0.3 -- 6 keV) instrument since 2008. We have developed a numerical time-depended code to calculate globally distributed flux (GDF) of hydrogen ENAs employing both 1) 3D kinetic-MHD model of the global heliosphere and 2) reconstruction of atom trajectories from 1 au, where they are observed by IBEX, to the point of their origin in the inner heliosheath (IHS). The key factor in the simulation is a detailed kinetic consideration of the pickup ions (PUIs), the supra-thermal component of protons in the heliosphere, which is "parental" to the ENAs and originates in the region of the supersonic solar wind being picked by the heliospheric magnetic field. As a result of our study, we have concluded that (1) the developed model is able to reproduce the geometry of the multi-lobe structure seen in the IBEX-Hi GDF maps, (2) the GDF is extremely sensitive to the form of the velocity distribution function of PUIs in the IHS, and the accounting for the existence of an additional energetic population of PUIs is essential to explain the data, (3) despite a relatively good agreement, there are some quantitative differences between the model calculations and IBEX-Hi data. Possible reasons for these differences are discussed.