Figure. Illustration of the processes involved in creating ribbon flux from the spatial retention model. The neutral interstellar wind flows into the heliosphere, creating pickup ions (PINI) in the solar wind and in the heliosheath. The neutral solar wind and neutral heliosheath atoms escape the heliosphere and ionize in the very local interstellar medium, creating more pickup ions (PINS and PINH, respectively). PINS and PINH created in the retention region can, after another charge-exchange, propagate back toward Earth and be detected by IBEX.
The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere, and after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbonʼs intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbonʼs source as well as an asymmetry in the ribbonʼs shape, i.e., a radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbonʼs shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbonʼs structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe may observe the ribbon.