Temporal evolution and rigidity dependence of the solar modulation lag of Galactic cosmic rays

Abstract

When traveling in the heliosphere, Galactic cosmic rays (GCRs) are subjected to the solar modulation effect, a quasiperiodical change of their intensity caused by the 11-year cycle of solar activity. Here we investigate the association of solar activity and cosmic radiation over five solar cycles, from 1965 to 2020, using a collection of multichannel data from neutron monitors, space missions, and solar observatories. In particular, we focus on the time lag between the monthly sunspot number and the GCR flux variations. We show that the modulation lag is subjected to a 22-year periodical variation, ranging from about 2 to 14 months and following the polarity cycle of the Sun's magnetic field. We also show that the lag is remarkably decreasing with increasing energy of the GCR particles. These results reflect the interplay of basic physics phenomena that cause the GCR modulation effect: the drift motion of charged particles in the interplanetary magnetic field, the latitudinal dependence of the solar wind, the energy dependence of their residence time in the heliosphere. Based on this interpretation, we end up with a global effective formula for the modulation lag and testable predictions for the flux evolution of cosmic particles and antiparticles over the solar cycle.