Correlation of the cosmic‐ray intensity with solar‐terrestrial parameters

Shea and Smart (1981) have shown that the correlation between the Mt. Washington neutron monitor counting rate and the geomagnetic aa index changes significantly from the eleven‐year period centered on one solar minimum to the next such period. They suggested that this may be a manifestation of cosmic‐ray drift in the interplanetary magnetic field. This Letter reports the results of numerical simulations of cosmic‐ray modulation, including drift, which verify that drifts can indeed produce the sense of the observed effect. Perturbations in the heliospheric equatorial regions affect galactic cosmic rays observed near the equator more effectively if the northern hemisphere heliospheric magnetic field is inward than if it is outward. Perturbations in K_⊥ are substantially more effective than perturbations in the solar wind velocity or in K_∥.