A technique for the study of short term cosmic ray variations : II) The Forbush Decrease

In this paper we present the results of applying the origin-of-scatter technique (see Paper I of this series) to study the Forbush decrease. A simple step decrease in cosmic ray number density which propagates outwards from the Sun is chosen to represent the situation in interplanetary space at the time of such events. The origin-of-scatter technique is shown to yield "synthetic" Forbush decreases similar to those observed in practice. It is found that the form of the modulation associated with the decrease is strongly dependent on the effective radial velocity of the number: density step, the scattering mean free path in interplanetary space, and the geographic longitude of the observing station-The longitude dependence of the modulation implies large anisotropies at Earth in agreement with observation. The modulation is greatest when the number density step has a very slow effective approach velocity and it is suggested that events showing strong modulation imply that the cosmic ray screening disturbance has an elongated envelope. In addition it is concluded that north-south anisotropies will appear at times of Forbush decreases if the large scale interplanetary magnetic field is inclined to the Earths equatorial pIane.