What the 4.3 - 8 8 keV ENA flux reveals about termination shock pickup ion spectra and heliospheric structure?

Energetic neutral atoms (ENAs) with the energies 4.7 - 88 keV, observed by IBEX-Hi, INCA, and HSTOF, trace the transmission and acceleration of pickup ions (PUIs) at the heliospheric termination shock (HTS) and their propagation within the inner heliosheath (IHS). The primordial seed of these ENAs are interstellar neutral (ISN) atoms entering the heliosphere. These atoms are ionized by charge exchange with solar wind protons and photoionization by solar EUV photons, and form the PUI population inside the HTS. At the HTS, some PUIs are transmitted directly into the IHS, and those with the energies below the potential jump are reflected into the supersonic SW and gain energy. These reflections/accelerations continue until the ions gain a sufficiently high energy to penetrate inside the IHS. Together with the transmitted PUIs, they are subsequently carried away by the ambient plasma and gradually de-charged by charge exchange with ambient interstellar H atoms, forming ENAs responsible for the global distributed flux. Some of these ENAs are detected deep inside the HTS. We verify our understanding of the above chain of processes by comparing selected measurements with model ENA fluxes. We simulate the distribution of the ENA flux in the ecliptic plane starting from the Warsaw (nWTPM) hot model of ISN H distribution inside the heliosphere using observation-based time- and heliolatitude evolution of the solar factors. For selected solar cycle phases, based on the simulated ISN H distribution between the Sun and HTS in the ecliptic, we calculate the PUI density at the HTS. We then use the Zank acceleration theory of the transmission and reflection of PUI components at the HTS to obtain the PUI spectra just beyond the HTS. We gradually decharge these ions as they propagate within the IHS plasma. We use the heliosheath distribution of ISN H, the HTS location, and the plasma flow from the Huntsville heliosphere model, run with the currently best parameters of interstellar gas, interstellar magnetic field, and solar factors. Finally, we simulate the ENA flux along the ecliptic in the energy bands corresponding to IBEX, INCA, and HSTOF measurements, to obtain a fairly good agreement with these measurements. This result is consistent with the comet-like image of the heliosphere.