Escape of Magnetic Flux Ropes from the Siege of Closed Magnetic Fields in the Solar Corona

Coronal mass ejections (CMEs) are believed to eject a global scale flux rope, which can even reach the earth's orbit and beyond. A great hindrance for its escape from the sun is strongly linetied, closed magnetic fields surrounding the flux rope. For a paramagnetic flux rope in a bipolar configuration to escape from this siege, most line-tied fields surrounding it need to reconnect, which can hardly take place. A diamagnetic flux rope is expected to escape from the siege of closed fields as long as it has more magnetic flux than the overlying field. In this paper, the evolution of a coronal magnetic field system in response to emergence of a diamagnetic flux rope is investigated by using numerical magnetohydrodynamic (MHD) simulations. The toroidal magnetic field of the emerging flux rope is set to be either parallel (case 1) or antiparallel (case 2) to the toroidal field of the overlying arcade. In case 1, magnetic reconnection between the emerging field and the overlying arcade field creates a new paramagnetic flux rope. Although the presence of this paramagnetic flux rope slows down reconnection between the overlying field and the emerging field in the early stage, the flux rope gathers more and more flux, expands and rises with time. In case 2, magnetic reconnection efficiently progresses from the beginning between the emerging diamagnetic flux rope and the overlying arcade field. This reconnection process removes not only the closed field barrier surrounding the diamagnetic flux rope, but also the poloidal flux in this flux rope. Thus, the flux rope can eventually become free to go indefinitely away, but with only a small flux in it. These two types of flux rope ejections may account for the different types of CMEs.