Solar-flare produced ³He in lunar samples

Abstract

Lunar rocks and soils undergo long-term irradiation by solar and galactic cosmic rays (SCR and GCR) leading to compositionally-distinct spallation spectra for noble gases^1,2 and leaving behind fossil records characteristic of different irradiation features. Because the SCR effects are restricted to the first few millimetres of rocks and soils in contrast to GCR, various studies based on solar flare tracks^3,4, thermoluminescence⁵, and radionuclides^3,6 over the past decade have aimed to decipher these fossil records bearing the imprints of the low (SCR) and high (GCR) energy irradiation phenomena. These studies have greatly helped our understanding of the energy spectra and fluxes of charged particles streaming out of the Sun. We have already reported quantitative studies, aimed at isolating the SCR and GCR produced Ne and Ar isotopes in several lunar soils and rocks^1,2 and have calculated their solar flare exposure ages which agree with the ‘surface’ exposure ages based on ²⁶Al and particle track studies³. We describe here, for the first time, using ³He in samples from three different depths of lunar rocks, a method for deducing the absolute SCR (flare) proton fluxes in the past few million years which compare favourably with those derived from ²⁶Al and ⁵³Mn studies^3,6 of other lunar samples.