Progressive transformation of a flux rope to an ICME

The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by the interplanetary coronal mass ejections (ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic flux and helicity which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 and 10 November, 2004. After determining an approximated orientation for the flux rope using the minimum variance method, we precise the orientation of the cloud axis relating its front and rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic flux between the in- and out-bound branches, and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed by an extended coherent magnetic region. These observations are interpreted considering the existence of a previous larger flux rope, which partially reconnected with its environment in the front. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed to the observed ICME (with a remnant flux rope in the front part).Comment: Solar Physics (in press