Determination of the flux and energy distribution of energetic solar protons in the past 2 Myr using lunar rock 68815

Abstract

Cosmogenic 21Ne, 22Ne, 38Ar, and 3He produced by nuclear interactions of energetic (>10 MeV) solar protons were determined in 11 depth samples of lunar rock 68815. Concentrations of these proton-produced, SCR nuclides smoothly decrease from the rock surface down to 4.3 cm, where a galactic cosmic ray (GCR) component dominates. The cosmogenic 21Ne 22Ne isotopic ratio shows a systematic change with depth, from the characteristic GCR value of 0.79 at 4.9 cm to a mixed SCR + GCR ratio of 0.66 near the surface. The surface exposure age for 68815 calculated from both SCR and GCR components for 21Ne, 22Ne, and 38Ar agrees with the reported 81Kr-Kr exposure age of 2.04 Myr. Theoretical SCR + GCR depth profiles were calculated from cross-section data for different assumed spectra of energetic solar protons ( R 0, or rigidity, values of 50–125 MV) and for different rock surface erosion rates (0–3 mm/Myr). These theoretical SCR profiles were added to GCR profiles and statistically compared with measured data by minimizing the standard deviation of the least squares statistical fit and by requiring the GCR component in the 4.3 cm sample to be >90% of the measured concentration. SCR 21Ne, 22Ne, and 38Ar give the following results for energetic solar protons over the last 2 Myr. For a preferred erosion rate of 1 mm/Myr and R 0 of 80–90 MV, J(4 π, E > 10 MeV) − 58–73 p/cm 2/ s; for the broader possible range of R 0 values of 70–100 MV, J(4 π, E > 10 MeV) − 49–92 p/cm 2/ s. These proton fluxes increase by ~ 12 and ~ 24% for erosion rates of 2 mm/Myr and 3 mm/Myr, respectively. These same data analysis techniques were also applied to literature depth profiles for SCR radionuclides in rock 68815. 26Al and 53Mn give proton fluxes comparable to those obtained from neon and argon at R 0 values of 70–85 MV, but give somewhat higher J values at larger values of R 0. Proton fluxes characterized by 81Kr and 14C tend to be higher compared to the other nuclides. The most likely reason for different J values determined from different SCR nuclides is errors in cross section data, although measurement errors and temporal variations in J cannot be excluded.