Quantifying the High Early Solar Cosmic-Ray Flux with Cosmogenic Neon Isotopes in Refractory Minerals

Abstract An enhancement in the activity of the early young Sun resulting in a high charged particle flux has been invoked to explain excesses in spallation-induced nuclides in primitive planetary materials. Astronomical observations of energetic outbursts of young stellar objects (YSOs) also support the idea of an active young Sun. However, the early solar cosmic-ray (SCR) flux has not been well constrained. Here we use measured concentrations of SCR-produced nuclides that formed and are preserved in meteoritical hibonite and spinel, some of the solar system’s oldest solids, and physical models for dust transport in the early protoplanetary disk to determine the magnitude of the early SCR flux. We focus our attention on cosmogenic neon, which cannot have been inherited from precursors and can only be produced in situ in solids. Our modeled effective exposure time to SCRs for these solids is very short, on the order of years. This indicates that the young Sun’s SCR flux recorded in refractory mineral hibonite was up to ∼7 orders of magnitude higher than the contemporary level. Our flux estimate is consistent with the >10 5 × enhanced flux inferred from astronomical observations of greatly enhanced flare activities of YSOs.