Forbush decreases (Fds) in galactic cosmic ray intensity are related to interplanetary coronal mass ejections (ICMEs). The parallel diffusion of particles is reduced because the magnetic turbulence level in sheath region bounded by ICMEs leading edge and shock is high. Besides, in sheath and magnetic cloud (MC) energetic particles would feel enhanced magnetic focusing effect caused by the strong inhomogeneity of the background magnetic field. Therefore, particles would be partially blocked in sheath-MC structure. Here, we study two-step Fds by considering the magnetic turbulence and background magnetic field in sheath-MC structure with diffusion coefficients calculated with theoretical models, to reproduce the Fd associated with the ground-level enhancement event on 2000 July 14 by solving the focused transport equation. The sheath and MC are set to spherical caps that are portions of spherical shells with enhanced background magnetic field. Besides, the magnetic turbulence levels in sheath and MC are set to higher and lower than that in ambient solar wind, respectively. In general, the simulation result conforms to the main characteristics of the Fd observation, such as the pre-increase precursor, amplitude, total recovery time, and the two-step decrease of the flux at the arrival of sheath and MC. It is suggested that sheath played an important role in the amplitude of Fd while MC contributed to the formation of the second step decrease and prolonged the recovery time. It is also inferred that both magnetic turbulence and background magnetic field in sheath-MC structure are important for reproducing the observed two-step Fd.
Download