Magnetic Cloud And Sheath In The Ground-Level Enhancement Event Of 2000 July 14. Ii. Effects On The Forbush Decrease

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 the sheath region bounded by the ICME's leading edge and shock is high. In the sheath and magnetic cloud (MC) energetic particles would feel an enhanced magnetic focusing effect caused by the strong inhomogeneity of the background magnetic field. Therefore, particles would be partially blocked in the sheath-MC structure. Here, we study two-step Fds by considering the magnetic turbulence and background magnetic field in the sheath-MC structure with diffusion coefficients calculated using 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. The magnetic turbulence levels in the sheath and MC are set to higher and lower than those 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 two-step decrease of the flux at the arrival of the sheath and MC. It is suggested that the sheath plays an important role in the amplitude of the Fd while the MC contributes to the formation of the second-step decrease and prolonged recovery time. It is also inferred that both magnetic turbulence and background magnetic field in the sheath-MC structure are important for reproducing the observed two-step Fd.