The Relationship of Magnetic Twist and Plasma Motion in a Magnetic Cloud


Our recent investigations indicate that interplanetary magnetic clouds (MCs) have a high-twist core and a weak-twist outer shell. Utilizing the velocity-modified uniform-twist force-free flux rope model, we further investigate the relationship between the twist profile of magnetic field lines and the distribution of the plasma poloidal angular velocity inside an MC. The poloidal velocity in the MC is 11 km s−1. There are evidently positive correlations between the absolute value of the twist and the plasma poloidal angular velocity in peeled flux ropes or flux rope layers, although the correlation coefficients in flux rope layers are less than those in peeled flux ropes. This finding suggests that plasma flows are frozen-in magnetic field lines as we expected for interplanetary medium, of which the magnetic Reynolds number is large. Furthermore, based on this picture, we infer the axial velocity in the MC frame, which is less than 10 km s−1 and almost uniform in the cross section of the MC. Besides, it is inferred that the plasma flows velocity in the MC is much less than the local Alfvén speed.

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The Astrophysical Journal



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