Large‐scale solar modulation of >̃ 500 MeV/nucleon galactic cosmic rays seen from 1 to 30 AU

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Abstract
Using measurements of ≳500 MeV/nucleon cosmic rays obtained by Cerenkov counters on Pioneer 10 and Pioneer 11 and neutron monitor data from earth, we can observe the spatial and temporal development of cosmic ray modulation during the last solar maximum. The large‐scale features of modulation and recovery are similar at these three sites and thus appear rotationally symmetric near the ecliptic plane. Outward propagating features characterize the radial dependence. The decline of the old cosmic ray cycle is marked by steplike decreases that propagate outward at nearly the solar wind velocity, as pointed out by other investigators. During the start of the new cosmic ray cycle, recovery occurs first in the inner heliosphere and, after a lag comparable with that of the declining phase, appears later farther out. However, the direction of diffusive propagation is still inward because the gradient remains positive. Forbush decreases are common at all three sites and are evidently of great importance in understanding modulation. The largest decrease occurred during a short series of events in summer 1982 and had half the amplitude of the 11‐year cycle.