Constraints on the IBEX Ribbon's Origin from its Evolution over a Solar Cycle

Abstract

The heliosphere surrounding our solar system is formed by the interaction between the solar wind and the local interstellar medium as the Sun moves through interstellar space. With dimensions on the order of hundreds to potentially thousands of au, it is difficult to determine the 3D structure of the heliosphere and the properties of the plasma surrounding it. However, observations of energetic neutral atoms (ENAs), which are created as a product of charge exchange between interstellar neutrals and energetic plasma ions, allow us to remotely discern the properties of the distant heliospheric boundaries.NASA's Interstellar Boundary Explorer (IBEX) mission, a small explorer spacecraft which has been measuring ENA fluxes at ~0.5-6 keV for more than a solar cycle, discovered a “ribbon” of enhanced ENA fluxes forming a narrow, circular band across the sky. While it is generally believed that the ribbon is formed from secondary ENAs from outside the heliopause, a source of ENAs that is separate from the globally distributed flux (GDF) across the sky, it is not clear exactly how the parent ions outside the heliopause evolve over time before they become ribbon ENAs. To help solve this issue, we present recent developments in modeling the evolution of the ribbon over a solar cycle, under different pitch angle scattering assumptions. We hypothesize that simulating the evolution of the ribbon under differing scattering rates may help discern the physical mechanisms responsible for creating the ribbon observed by IBEX. We compare our modeling results to IBEX data where the ribbon was separated from the GDF using spherical harmonic decomposition, analyzing the evolution of the ribbon’s intensity and geometry as a function of ENA energy.