Solar Activity over the Last 10 Million Years

The depth profiles of cosmogenic isotopes in the lunar regolith depend on the flux and spectrum of Galactic and solar cosmic rays (GCRs and SCRs) and, therefore, depend on solar activity on a time scale comparable to the lifetime of these isotopes. In this work, we analyzed the content of various radionuclides (¹⁴C, ²⁶Al, ¹⁰Be, and ⁵³Mn) in samples obtained by the Apollo 15 mission. Comparing the results of modeling performed for the average GCR flow using the GEANT4.10 package with experimental data, we obtained a correction factor for the calculated formation rates of Y₀ ~ 0.6 for all the considered radionuclides. We attribute this result to the overestimated value of the flux of secondary particles in the lunar soil in the calculation using the GEANT4.10 package. This conclusion is supported by independent laboratory experiments. The estimated ¹⁰Be depth profile can be consistent with the experimental data only if the additional (apart from the GCR) contribution of protons accelerated on the shock wave from a nearby supernova ~2.5 million years ago is taken into account. We also calculated the ⁵³Mn depth profile (with the longest half-life of those we considered), which can also be described taking the contribution of the supernova into account. We note that three long-lived isotopes, ²⁶Al, ¹⁰Be, and ⁵³Mn, with different half-lives were modeled with the same average modulation potential. This allowed us to conclude that solar activity did not undergo noticeable changes on a time scale of about 10 million years.