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Ground-level enhancements (GLEs) generally accompany with fast interplanetarycoronal mass ejections (ICMEs), the shocks driven by which are the effectivesource of solar energetic particles (SEPs). In the GLE event of 2000 July 14,observations show that a very fast and strong magnetic cloud (MC) is behind theICME shock and the proton intensity-time profiles observed at 1 au had a rapidtwo-step decrease near the sheath and MC. Therefore, we study the effect ofsheath and MC on SEPs accelerated by an ICME shock through numerically solvingthe focused transport equation. The shock is regarded as a moving source ofSEPs with an assumed particle distribution function. The sheath and MC are setto thick spherical caps with enhanced magnetic field, and the turbulence levelsin sheath and MC are set to be higher and lower than that of the ambient solarwind, respectively. The simulation results of proton intensity-time profilesagree well with the observations in energies ranging from $\sim$1 to $\sim$100MeV, and the two-step decrease is reproduced when the sheath and MC arrived atthe Earth. The simulation results show that the sheath-MC structure reduced theproton intensities for about 2 days after shock passing through the Earth. Itis found that the sheath contributed most of the decrease while the MCfacilitated the formation of the second step decrease. The simulation alsoinfers that the coordination of magnetic field and turbulence in sheath-MCstructure can produce a stronger effect of reducing SEP intensities.

Magnetic Cloud and Sheath in the Ground-Level Enhancement Event of 2000 July 14. I. Effects on the Solar Energetic Particles