The energetic particle intensity estimated from cosmogenic isotope Al-26 produced in lunar samples

Abstract

Direct measurements of solar energetic particles (SEP) became possible only in the space era covering past several decades. However, for many academic and practical reasons, it is important to know the SEP energy spectrum on much longer time scales in the past. Such information can be obtained using reconstructions based on cosmogenic radioisotope measurements in extra-terrestrial objects without magnetic and atmospheric shielding such as lunar surface or me-teoroids. Thanks to the Apollo missions, samples of lunar rocks have been brought to the Earth and measured for the isotope content. Although estimates of the average SEP energy spectrum from cosmogenic radionuclides measured in lunar samples have been made earlier, here we revisit the approach using newly calculated depth profile of the yield function for 26Al in lunar rocks. We have developed a full Monte-Carlo model of the nuclide production by energetic particles in the rock. As a result, we present the improved estimate of the average solar energetic particle intensity at 1 AU on the multi-millenial time scales.