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

TL;DR

This study introduces a velocity-modified cylindrical flux rope model to analyze plasma motion in magnetic clouds at 1 AU, revealing significant propagation, expansion, and poloidal motions, and their implications for CME evolution.

Contribution

The paper develops a new velocity-modified flux rope model incorporating global motions, providing novel insights into plasma dynamics of magnetic clouds at 1 AU.

Findings

Some MCs exhibit significant perpendicular propagation velocities.

Most MCs do not expand self-similarly at 1 AU.

A notable poloidal motion exists in some MCs, with possible causes discussed.

Abstract

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