The Twist Profile in the Cross Section of Interplanetary Magnetic Clouds

Magnetic flux ropes (MFRs) as a well-organized magnetic field structure embedded in space plasmas have been widely studied for several decades. The twists of magnetic field lines in MFRs can yield much information regarding the formation and stability of MFRs, yet there is still open debate about them. Here, with the aid of a uniform-twist force-free flux rope model, we study the twist profile in the cross section of a interplanetary magnetic cloud (MC) by peeling off equal azimuthal magnetic flux layer by layer from the outermost shell, just like peeling an onion. The absolute value of the average twist, $\overline{\tau }$, and the twist in each layer, τ, exhibit an almost monotonous decrease from the axis to the periphery of the MC, but τ has a larger relative error. However, they do have a coincident trend of a high-twist core and an low-twist outer shell. The twist number per unit length, $\overline{\tau }$/τ, follows a linear trend versus $\tfrac{1}{\pi R}$, where R is the radius of each layer, with a correlation coefficient of 0.96/0.91 and slope of 0.27/0.26, which is well below the critical slope of 1 suggested by Wang et al.