The internal structure of coronal mass ejections: Are all regular magnetic clouds flux ropes?

Abstract

In this Letter, we investigate the internal structure of a coronal mass ejection (CME) and its dynamics by invoking a realistic initiation mechanism in a quadrupolar magnetic setting. The study comprises a compressible three-dimensional magnetohydrodynamics simulation. We use an idealized model of the solar corona, into which we superimpose a quadrupolar magnetic source region. By applying shearing motions resembling flux emergence at the solar boundary, the initial equilibrium field is energized and it eventually erupts, yielding a fast CME. The simulated CME shows the typical characteristics of a magnetic cloud (MC) as it propagates away from the Sun and interacts with a bimodal solar wind. However, no distinct flux rope structure is present in the associated interplanetary ejection. In our model, a series of reconnection events between the eruptive magnetic field and the ambient field results in the creation of significant writhe in the CME's magnetic field, yielding the observed rotation of the magnetic field vector, characteristic of an MC. We demonstrate that the magnetic field lines of the CME may suffer discontinuous changes in their mapping on the solar surface, with footpoints subject to meandering over the course of the eruption due to magnetic reconnection. We argue that CMEs with internal magnetic structure such as that described here should also be considered while attempting to explain in situ observations of regular MCs at L1 and elsewhere in the heliosphere. © 2009 The American Astronomical Society. All rights reserved.