Narrowing Down the Physical Mechanisms Governing the IBEX Ribbon

The Interstellar Boundary Explorer (IBEX) measures energetic neutral atom (ENA) emissions from the outer heliosphere. It observes a broad distribution of ENA fluxes known as the "globally distributed flux" (GDF) originating from the inner heliosheath, a region of hot solar wind plasma embedded with energetic pickup ions (PUIs). IBEX also observes a narrow band of enhanced ENA emission called the "ribbon". While the ribbon's origin remained elusive for years, with many theories proposed to explain where it came from, it is now generally accepted that it is formed by the secondary ENA mechanism. Secondary ENAs are generated by neutralization of PUIs gyrating around the interstellar magnetic field (ISMF) draped around the heliosphere in the outer heliosheath. However, before these PUIs become ENAs, they are strongly affected by kinetic processes such as pitch angle scattering, diffusion, and large-scale gradients in the ISMF, all of which influence the shape and structure of the IBEX ribbon. We present an overview of recent developments aimed at explaining how different assumptions for the physical mechanisms controlling PUIs in the outer heliosheath affect the IBEX ribbon's shape and structure. We present simulations of the ribbon assuming different levels of pitch angle scattering (weak, intermediate, strong) where PUIs are either free to propagate along magnetic field lines, are trapped by magnetic mirroring near B·r = 0 but maintain highly anisotropic distributions, or experience strong pitch angle scattering known as spatial retention. The geometry of the IBEX ribbon, namely its angular radius and center, and its cross-sectional structure are strongly affected by these assumptions. By quantifying the most essential observables of the IBEX ribbon, we show which physical assumptions appear most likely to be responsible for the creation of the ribbon.