US EPA

4967956 

Journal Article 

Understanding the Twist Distribution Inside Magnetic Flux Ropes by Anatomizing an Interplanetary Magnetic Cloud 

Wang, Y; Shen, C; Liu, Rui; Liu, J; Guo, J; Li, X; Xu, M; Hu, Q; Zhang, T 

2018 

123 

5 

3238-3261 

10.1002/2017JA024971 

WOS:000435943300001 

Magnetic flux rope (MFR) is the core structure of the greatest eruptions, that is, the coronal mass ejections (CMEs), on the Sun, and magnetic clouds are posteruption MFRs in interplanetary space. There is a strong debate about whether or not a MFR exists prior to a CME and how the MFR forms/grows through magnetic reconnection during the eruption. Here we report a rare event, in which a magnetic cloud was observed sequentially by four spacecraft near Mercury, Venus, Earth, and Mars, respectively. With the aids of a uniform-twist flux rope model and a newly developed method that can recover a shock-compressed structure, we find that the axial magnetic flux and helicity of the magnetic cloud decreased when it propagated outward but the twist increased. Our analysis suggests that the "pancaking" effect and "erosion" effect may jointly cause such variations. The significance of the pancaking effect is difficult to be estimated, but the signature of the erosion can be found as the imbalance of the azimuthal flux of the cloud. The latter implies that the magnetic cloud was eroded significantly leaving its inner core exposed to the solar wind at far distance. The increase of the twist together with the presence of the erosion effect suggests that the posteruption MFR may have a high-twist core enveloped by a less-twisted outer shell. These results pose a great challenge to the current understanding on the solar eruptions as well as the formation and instability of MFRs. 

magnetic flux ropes; coronal mass ejections; magnetic clouds; instabilities; twist