Response of Pickup Ions in the Very Local Interstellar Medium to Solar Variations: Implications for the Evolution of the IBEX Ribbon and Interstellar Helium

The Interstellar Boundary Explorer (IBEX) observes the "ribbon" of enhanced energetic neutral atom (ENA) fluxes from the outer heliosphere. The ribbon flux is likely formed from the neutralization of energetic pickup ions (PUIs) gyrating in the interstellar magnetic field outside the heliopause. Voyager 1 crossed the heliopause in 2012 and has observed several shocks in the very local interstellar medium (VLISM) that likely originate from merged interaction regions in the inner heliosphere that propagated outside the heliopause. We simulate the response of PUIs and the IBEX ribbon flux to solar disturbances propagating into the VLISM. First, we show that PUIs outside the heliopause respond significantly to the dynamic neutralized solar wind (SW) via charge exchange and to interactions with shocks via adiabatic heating/cooling. However, the evolution of ribbon fluxes at 1 au is primarily driven by changes in the neutralized SW and not PUI interactions with shocks outside the heliopause. Comparisons with IBEX observations of the ribbon at 1.1 keV show that an abrupt decrease in ENA fluxes observed in 2012 was caused by a drop in SW (and thus neutralized SW) speed by ∼100 km s^-1. Our simulation predicts a recovery of 1.1 keV ribbon fluxes starting in 2019 to levels observed early in the mission owing to an increase in SW speed. We also estimate that the presence of interstellar helium in the VLISM reduces the effectiveness of charge-exchange sources for PUIs and reduces the model ribbon flux at 1 au by ∼40%, matching well with IBEX ribbon fluxes.