Insights into the Heliospheric Energetic Neutral Atom Source with Modeling

Abstract:

The observation of energetic neutral atom (ENA) fluxes by the Interstellar Boundary Explorer (IBEX) has been a fruitful venture in helping us understand the global structure and thermodynamic properties of the heliosphere. With measurements spanning over a complete solar cycle, we can quantify the evolution of ENA fluxes and their parent pickup ion (PUI) distributions over a wide range of solar wind conditions.

Non-thermal PUIs are born through the ionization of interstellar neutral atoms flowing into the heliosphere from the local interstellar medium (LISM), forming a suprathermal ion component embedded within the solar wind. Charge-exchange of these PUIs beyond the heliospheric termination shock generates a primary source of ENAs, enabling IBEX to remotely probe the heliosheath, its structure, and the source PUI distribution. PUIs are also born by charge-exchange from ENAs that propagated outside the heliosphere and into the LISM, forming the parent seed population of the narrow but bright “ribbon,” a secondary source of ENAs.

Due to the complexities in the interactions of PUIs with shocks, turbulence, large-scale flows and pressure changes, and the inherent line-of-sight integration of measured ENA fluxes, sophisticated modeling techniques are necessary to reveal the ENA source characteristics. I will present recent developments in modeling the source of ENAs observed by IBEX from within the heliosheath and outside the heliopause, and our current understanding of the behavior of PUIs in the outer heliosphere deduced from IBEX measurements. Furthermore, unanswered key questions about the heliospheric ENA signal will be discussed in light of the upcoming Interstellar Mapping and Acceleration Probe (IMAP) mission.