On the Energization of Pickup Ions Downstream of the Heliospheric Termination Shock by Comparing 0.52–55 keV Observed Energetic Neutral Atom Spectra to Ones Inferred from Proton Hybrid Simulations

Abstract

We present an unprecedented comparison of ∼0.52–55 keV energetic neutral atom (ENA) heliosheath measurements, remotely sensed by the Interstellar Boundary Explorer (IBEX) mission and the Ion and Neutral Camera (INCA) on the Cassini mission, with modeled ENAs inferred from interstellar pickup protons that have been accelerated at the termination shock, using hybrid simulations, to assess the pickup ion energetics within the heliosheath. This is the first study to use hybrid simulations that are able to accurately model the acceleration of ions to tens of keV energies, which is essential in order to model ENA fluxes in the heliosheath, covering the full energy range observed by IBEX and CASSINI/INCA. The observed ENA intensities are an average value over the time period from 2009 to the end of 2012, along the Voyager 2 (V2) trajectory. The hybrid simulations upstream of the termination shock, where V2 crossed, are constrained by observations. We report an energy-dependent discrepancy between observed and simulated ENA fluxes, with the observed ENA fluxes being persistently higher than the simulated ones. Our analysis reveals that the termination shock may not accelerate pickup ions to sufficient energies to account for the observed ENA fluxes. We, thus, suggest that the further acceleration of these pickup ions is most likely occurring within the heliosheath, via additional physical processes like turbulence or magnetic reconnection. However, the redistribution of energy inside the heliosheath remains an open question.

An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

“…Giacalone & Decker (2010) show that such simulations produce a good agreement with the lower energy channels of Voyager 2 LECP measurements. Gkioulidou et al (2022) used an ENA model with a combination of different ion populations to reproduce the hybrid ion fluxes of Giacalone et al (2021) for the Voyager 2 direction. The ion distribution downstream of the termination shock was treated as two separate populations: transmitted PUIs with a kappa distribution and a combined reflected and accelerated PUI population with a kappa distribution.…”

“…The authors found that the modeled ENAs, for their best-fit model, differed from the observed ENAs of IBEX-Hi and INCA by an energy-dependent scaling factor. The discrepancy between modeled and observed ENA flux seen in other ENA models remained in the work of Gkioulidou et al (2022), despite improved modeling of the ion distribution downstream of the termination shock according to the hybrid model of Giacalone et al (2021). The authors concluded that some additional heating, or acceleration, of the PUI distribution in the heliosheath was necessary to replicate the data trend for all energies, especially at energies from 4 to 10 keV.…”

“…In this work, we expand on the work of Gkioulidou et al (2022), separating the reflected and accelerated PUIs, which Giacalone et al (2021) characterize as low-energy ACRs for energies >5 keV, to investigate the implications of PUI acceleration at the termination shock on ENAs. We model the best-fit ion distributions to the hybrid simulation of Giacalone et al (2021).…”

“…In Section 2, we discuss a modeling procedure based on a theoretical model by Wang et al (2023) to replicate the ion fluxes from Giacalone et al (2021) and present comparisons between our modeled distributions and the hybrid results. In Section 3, we apply our ion flux distributions in our ENA model to compare with IBEX-Hi and INCA observations as in Gkioulidou et al (2022). In Section 4, we present a summary of our findings.…”

Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

“…The GDF, believed to primarily originate from the IHS, exhibits power lawlike spectra (Zirnstein et al 2021a). However, thus far, the total intensity of the observed GDF has proven challenging to recreate in models (Zirnstein et al 2017;Baliukin et al 2020;Fuselier et al 2021;Kornbleuth et al 2021;Gkioulidou et al 2022), suggesting the importance of turbulence at the HTS (Giacalone et al 2021;Zirnstein et al 2021b), an additional heating process in the IHS (Chalov et al 2003;Zirnstein et al 2018a), a separate ENA source from outside the heliopause (Izmodenov et al 2009;Opher et al 2013;Zirnstein et al 2014), or some combination of these. We address this issue by utilizing a dynamic model of PUI transport through the heliosphere to simulate the origin and evolution of IBEX ENA observations.…”