Investigating plasma motion of magnetic clouds at 1 AU through a velocity-modified cylindrical force-free flux rope model

摘要

Magnetic clouds (MCs) are the interplanetary counterparts of coronal massejections (CMEs), and usually modeled by a flux rope. By assuming thequasi-steady evolution and self-similar expansion, we introduce three types ofglobal motion into a cylindrical force-free flux rope model, and developed anew velocity-modified model for MCs. The three types of the global motion arethe linear propagating motion away from the Sun, the expanding and the poloidalmotion with respect to the axis of the MC. The model is applied to 72 MCsobserved by Wind spacecraft to investigate the properties of the plasma motionof MCs. First, we find that some MCs had a significant propagation velocityperpendicular to the radial direction, suggesting the direct evidence of theCME's deflected propagation and/or rotation in interplanetary space. Second, weconfirm the previous results that the expansion speed is correlated with theradial propagation speed and most MCs did not expand self-similarly at 1 AU. Inour statistics, about 62\%/17\% of MCs underwent a under/over-expansion at 1 AUand the expansion rate is about 0.6 on average. Third, most interestingly, wefind that a significant poloidal motion did exist in some MCs. Threespeculations about the cause of the poloidal motion are therefore proposed.These findings advance our understanding of the MC's properties at 1 AU as wellas the dynamic evolution of CMEs from the Sun to interplanetary space.