Solar proton fluxes during the last million years

The solar flare proton fluxes, averaged over different periods of time, have been determined from Al-26 measured in five selected lunar rocks. The time of exposure of these rocks, ranging from 0.5 to 3.8 million years, were deduced from track profiles due to galactic-cosmic-ray heavy nuclei and from rare-gas data. The observed production rates of Al-26 in all the rocks is in close agreement, implying little variation in average solar flare proton intensity during the past 0.5-, 1-, and 1.5-million-year periods. If the fluxes of solar flare protons and thermal radiation are related, these data exclude any significant changes in solar luminosity during the Pleistocene