A 22‐year dependence in the size of near‐ecliptic corotating cosmic ray depressions during five solar minima

Abstract

Evidence is presented for a 22‐year cycle in the amplitude of recurrent, near‐Earth, near‐Ecliptic, galactic cosmic ray modulations during the solar minimum periods in the mid‐1950s to mid‐1990s. These modulations are ∼50% larger during A > 0 epochs than during A < 0 epochs, where A is the direction of the solar global magnetic field. This difference is evident in all the cosmic ray data examined, including observations by neutron monitors and integral and differential particle data from the IMP 8 spacecraft. A change in the properties of the associated corotating streams does not appear to be responsible. Rather, the response of the cosmic rays to solar wind speed enhancements appears to be reduced in A < 0 minima. This pattern is inconsistent with a prediction from the drift model of cosmic ray modulation that recurrent modulations should be larger in A < 0 epochs. Other factors may contribute, such as an epoch‐dependence of the particle diffusion coefficient which may increase the effect of solar wind convection on the local cosmic ray density near Earth during A > 0 epochs. A change in the latitudinal GCR gradient in successive epochs is unlikely to be the sole reason for the change in depression size since (1) the proportionality constant between the recurrent depressions and latitudinal gradient depends on A , and (2) the cosmic ray density is not organized by crossings of the heliospheric current sheet.