We examine a three‐dimensional (3‐D) numerical magnetohydrodynamic (MHD) simulation describing a very fast interplanetary coronal mass ejection (ICME) propagating from the solar corona to 1 AU. In conjunction with its high speed, the ICME evolves in ways that give it a unique appearance at 1 AU that does not resemble a typical ICME. First, as the ICME decelerates far from the Sun in the solar wind, filament material at the back of the flux rope pushes its way forward through the flux rope. Second, diverging nonradial flows in front of the filament transport poloidal flux of the rope to the sides of the ICME. Third, the magnetic flux rope reconnects with the interplanetary magnetic field (IMF). As a consequence of these processes, the flux rope partially unravels and appears to evolve to an entirely unbalanced configuration. At the same time, filament material at the base of the flux rope moves forward and comes in direct contact with the shocked plasma in the CME sheath. We find evidence that such remarkable behavior has actually occurred when we examine a very fast CME that erupted from the Sun on 2005 January 20. In situ observations of this event near 1 AU show very dense cold material impacting the Earth following immediately behind the CME sheath. Charge state analysis shows this dense plasma is filament material. Consistent with the simulation, we find the poloidal flux ( B z ) to be entirely unbalanced, giving the appearance that the flux rope has eroded. The dense solar filament material and unbalanced positive IMF B z produced a number of anomalous features in a moderate magnetic storm already underway, which are described in a companion paper by Kozyra et al. (2014). Key Points Filament material can move to the front of ICMEs Flux rope erosion can occur by azimuthal transport of poloidal fluxPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108271/1/jgra51093.pd
