Solar Energetic Particle Track-production Rates at 1 au: Comparing In Situ Particle Fluxes with Lunar Sample-derived Track Densities

Abstract

Heavy (Z > 26) solar energetic particles (SEPs) with energies ∼1 MeV nucleon−1 are known to leave visible damage tracks in meteoritic materials. The density of such “solar flare tracks” in lunar and asteroidal samples has been used as a measure of a sample’s exposure time to space, yielding critical information on planetary space weathering rates, the dynamics and lifetimes of interplanetary dust grains, and the long-term history of solar particle fluxes. Knowledge of the SEP track accumulation rate in planetary materials at 1 au is critical for properly interpreting observed track densities. Here, we use in situ particle observations of the 0.50−3.0 MeV nuc−1 Fe-group SEP flux taken by NASA’s Advanced Composition Explorer (ACE) to calculate a flux of track-inducing particles at 1 au of 6.0 × 105 cm−2 yr−1 str−1. Using the observed energy spectrum of Fe-group SEPs, we find that the depth distribution of SEP-induced damage tracks inferred from ACE measurements matches closely to that recently measured in lunar sample 64455; however, the magnitude of the ACE-inferred rate is approximately 25× higher than that observed in the lunar sample. We discuss several hypotheses for the nature of this discrepancy, including inefficiencies in track formation, thermal annealing of lunar samples, erosion via space weathering processing, and variations in the SEP flux at the Moon, yet find no satisfactory explanation. We encourage further research on both the nature of SEP track formation in meteoritic materials and the flux of Fe-group SEPs at the lunar surface in recent and geologic times to resolve this discrepancy.