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

TL;DR

This study uses numerical simulations to analyze how sheath and magnetic cloud structures influence solar energetic particle profiles during a 2000 GLE event, revealing their roles in intensity decreases and the importance of magnetic and turbulence conditions.

Contribution

It introduces a focused transport model incorporating sheath and magnetic cloud effects, providing insights into SEP intensity variations during GLE events.

Findings

Sheath causes the most significant SEP intensity decrease.

Magnetic cloud facilitates the formation of a second intensity decrease.

Simulation results align well with observed proton profiles.

Abstract

Ground-level enhancements (GLEs) generally accompany with fast interplanetary coronal mass ejections (ICMEs), the shocks driven by which are the effective source 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 the ICME shock and the proton intensity-time profiles observed at 1 au had a rapid two-step decrease near the sheath and MC. Therefore, we study the effect of sheath and MC on SEPs accelerated by an ICME shock through numerically solving the focused transport equation. The shock is regarded as a moving source of SEPs with an assumed particle distribution function. The sheath and MC are set to thick spherical caps with enhanced magnetic field, and the turbulence levels in sheath and MC are set to be higher and lower than that of the ambient solar wind, respectively. The simulation results of proton intensity-time profiles agree well with the observations in energies ranging from $\sim$1 to $\sim$100 MeV, and the two-step decrease is reproduced when the sheath and MC arrived at the Earth. The simulation results show that the sheath-MC structure reduced the proton intensities for about 2 days after shock passing through the Earth. It is found that the sheath contributed most of the decrease while the MC facilitated the formation of the second step decrease. The simulation also infers that the coordination of magnetic field and turbulence in sheath-MC structure can produce a stronger effect of reducing SEP intensities.