A Brief History of Recurrent Solar Modulation of the Galactic Cosmic Rays (1937-1990)

This brief review of the pre-Ulysses era begins with the first measurements by ionization chambers in 1937 of a cosmic ray 27-day intensity variation that was believed to have its origin in recurrent variations of the geomagnetic field. However, with the introduction of neutron monitor analysis of the nucleonic component, it was shown in the 1940s and 1950s that this cosmic ray intensity variation arose from interplanetary dynamical phenomena. Beginning in the 1960s direct spacecraft investigations in the heliosphere with Pioneer-10, Pioneer-11, Voyager-1 and Voyager-2 proved that Corotating Interaction Regions were not only the source of the cosmic ray recurrent intensity modulation, but also the source of charged particles accelerated in corotating forward and reverse shocks associated with the corotating interaction regions.These early investigations, confined to low latitudes, have contributed to the understanding of solar phenomena, interplanetary dynamics, charge particle acceleration and the Sun-Earth convection.