Solar cycle variations and cosmic rays

Cosmic rays are excellent indicators of the various solar cycle variations. Galactic and anomalous cosmic rays encounter an outward moving solar wind with cyclic magnetic-field fluctuations and turbulence, which constitute the convection and diffusion processes in the heliosphere. They also lose energy as they propagate inwards to Earth, and experience current sheet, global curvature and gradient drifts in the heliospheric magnetic field. As a result, the intensity of cosmic rays directly reflects the various solar cycle variations, from the well-known 11- and 22-year cycles, with the reversal of the solar magnetic field at extreme solar maximum, to highly temporal variations like proton flares, Forbush decreases, corotating interaction regions and a variety of propagating diffusion barriers. All these features contribute and influence space climate and weather at Earth. Recently, the time-dependent extent and the dynamics of the heliosphere, in particular the role of the heliosheath and the location of the heliopause, have been emphasized as important to very long-term space climate. Long-term modulation over 11–22 years is breifly reviewed with emphasis on the compound time-dependent approach in modeling the solar cycle variations of cosmic rays in the heliosphere.