Solar energetic particles (SEPs) and Forbush decreases (Fds) in galactic cosmic ray (GCR) intensity are two important phenomena accompanied by interplanetary coronal mass ejections (ICMEs). A fast and strong magnetic cloud (MC) was behind the ICME-driven shock associated with the ground-level enhancement event on 2000 July 14. Observations show that both SEPs and GCRs had a rapid two-step decrease near the sheath and MC arrivals at 1 au. We therefore study the effect of sheath and MC on the SEPs and Fd by numerically solving the focused transport equation. In the simulation model, the MC and sheath are modeled as thick spherical caps behind the ICME shock with enhanced magnetic field. The magnetic turbulence levels in the MC and sheath are set to be lower and higher than that in the ambient solar wind, respectively. The simulated SEP intensity-time profiles fit the observations well in energies ranging from $\sim$1 to $\sim$100 MeV, and the simulated GCR intensity reproduces the main characteristics of the Fd, such as the pre-increase precursor, amplitude, total recovery time. It is found that the two-step decreases are reproduced at the sheath and MC arrivals and both the magnetic field and magnetic turbulence in the sheath-MC structure are important for the formation of the two-step decreases. It is suggested that the sheath produced most of the decrese while the MC contributed to the formation of the second step decrease for both the SEPs and Fd, and the MC also prolonged the recovery time of the Fd.
Solar energetic particle; Forbush decrease; galactic cosmic ray; coronal mass ejection; coronal mass ejection shock; space weather