Numerical evaluation of the precursory increase to the Forbush decrease expected from the Diffusion Convection Model

A model calculation of the cosmic ray Forbush decrease is performed on the basis of the diffusion‐convection model. The main purpose is to evaluate the magnitude of the precursory increase expected from a model where the cosmic ray particles are swept away by traveling disturbances in which the solar wind speed is raised and/or the diffusion coefficient is reduced. It is found that the ratio of the precursory increase to the main decrease is less than ∼10% if the field‐aligned diffusion coefficient is larger than ∼3×10 21 cm²/s at 1 AU and the speed of the solar wind disturbance is less than ∼1000 km/s. Existence of a small precursory increase of this magnitude does not seem to contradict the observations of the Forbush decrease. The precursory increase can be >20% of the main decrease when the speed of the disturbance is >2000 km/s, however, and August 4, 1972, event seems to represent such a case.