Image of Forbush Decrease in a Magnetic Cloud by Three Moments of Cosmic Ray Distribution Function

Abstract

AbstractThe time dynamics of the cosmic ray distribution function in a magnetic cloud is calculated. The magnetic cloud has the form of a torus segment with the force‐free magnetic field structure at the initial moment. The subsequent propagation of the magnetic cloud in interplanetary space is determined by inertial description. The magnetic field is determined by the frozen‐in condition. When calculating the particle distribution function, the electromagnetic field of the magnetic cloud is taken into account, with the scattering of particles not being taken into account. Relations between the particle distribution function and its three moments (particle density and unidirectional and bidirectional anisotropies) are derived. It is established that cosmic ray losses at the regions connecting the magnetic cloud with the Sun determine the amplitude of the second step of Forbush decrease. The time dependence of the Forbush decrease characteristics on magnetic cloud type is determined. The calculation results of the particle density and unidirectional anisotropy generally correspond to measurements. The referred results show a prominent role of the magnetic field structure in the time dynamics of the Forbush decrease.