Atmospheric radioactivity and variations in the solar neutrino flux

Abstract

Measurements¹ of the flux of neutrinos emitted as by-products of hydrogen fusion reactions in the Sun's core bear on questions ranging from stellar evolution theories^2,3 to the question of the neutrino mass. Attention has been devoted recently to possible time variations in the neutrino flux data, and some investigators have found an inverse correlation with the 11-year sunspot cycle^4–7, although there is still some debate about the statistical significance of this correlation⁸. The neutrino luminosity of the Sun is not expected to vary on timescales less than one million years, so alternative sources of neutrinos have been invoked to explain the temporal variations. Galactic cosmic rays have been considered to be a likely candidate, as their intensity at the Earth's orbit varies inversely with the activity of the Sun. Here we present a calculation of the inventory of positron emitters in the Earth's atmosphere that results from hadronic cascades initiated by galactic cosmic rays, and the variation of this source of low-energy neutrinos with the 11-year solar cycle. Our results rule out the possibility that cosmogenic neutrino emitters are responsible for an apparent solar-cycle dependence in Davis's solar neutrino experiment¹. As atmospheric secondary-particle decay was previously eliminated as a significant neutrino source⁹, any such variations, if corroborated over the next sunspot cycle, would appear to be caused by phenomena outside the Earth's atmosphere, most likely in the Sun itself.