On the energization of pickup ions downstream of the heliosheric termination shock, by comparing 0.52-55 keV observed ENA spectra to simulated ENAs inferred by proton hybrid simulations.
Abstract
As the solar system and its surrounding heliosphere move through the local interstellar medium, interstellar neutral atoms, mostly atomic Hydrogen, enter the heliosphere and undergo charge-exchange collisions with the continuously flowing solar wind protons. Newly created ions from the interstellar neutral population are advected outward with the solar wind, forming a population that is commonly known as pickup ions (PUIs). When PUIs reach the termination shock, they are heated, with a fraction of their distribution being reflected off the shock surface and undergoing additional heating. The heated PUIs that populate the heliosheath (HS), charge-exchange with the interstellar neutrals, creating Energetic Neutral Atoms (ENAs) that are measured remotely by the Interstellar Boundary Explorer (IBEX; 0.01-6 keV) and Cassini/Ion and Neutral Camera (INCA; 5.2-55 keV). Understanding the PUI distribution in the heliosheath is essential in order to i) study the pressure balance and acceleration mechanisms inside the heliosheath, and ii) to determine the ENA emission from the heliosheath, since these ENAs are used to remotely sense the boundaries of our heliosphere and its interaction with the very local interstellar medium. In this study, 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 ENA inferred from interstellar pickup protons that have been accelerated at the termination shock using hybrid simulations, towards assessing the PUI energetics within the heliosheath. This is the first study to use hybrid simulations that are able to accurately model the acceleration of ions to 10s 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 trajectory. The hybrid simulations upstream of the termination shock, where Voyager 2 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 pick up ions to sufficient energies to account for the observed ENA fluxes. We, thus, suggest that the further acceleration of these pick up ions is most likely occurring within the heliosheath, via additional physical processes like turbulence or magnetic reconnection. Yet, the redistribution of energy inside the heliosheath remains an open question.
- Publication:
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44th COSPAR Scientific Assembly. Held 16-24 July
- Pub Date:
- July 2022
- Bibcode:
- 2022cosp...44.1315G