Topic/Type: 1.1 Space & astrophysical plasmas, Oral

Disentangling the internal structure of a coronal mass ejection.

C. Jacobs1, I. Roussev2, N. Lugaz2, S. Poedts1

1 Centre for Plasma-Astrophysics, K.U.Leuven, Leuven, Belgium
2 Institute for Astronomy, University of Hawaii, Honolulu, U.S.A.

Coronal mass ejections (CMEs) are one of the most violent events occurring on the Sun. During a CME, a huge amount of solar material (of the order g) is ejected into interplanetary space on a time scale of only a few hours. CMEs propagate on average with velocities of 400-500 km/s, but speeds of more than 2000 km/s have been measured as well. Especially the fast events are interesting to study since they cause shock waves, which can accelerate particles to very high energies. When such a solar eruption is directed towards the Earth, the associated shock wave, energetic particles, and plasma cloud can interact with the magnetosphere of the Earth. The geo-effectiveness of a CME is largely determined by the orientation of the magnetic field inside the associated plasma cloud (also referred to as magnetic cloud (MC)). When this field is of the opposite orientation of the Earth\'s magnetic field, CMEs can cause severe geo-magnetic storms which can have harmful consequences for communication and navigation systems, power supplies, etc. Therefore, CMEs are considered as one of the most important drivers of the space weather. A clear understanding of the origin, the structure, and the propagation characteristics of those violent solar phenomena is essential for a deeper insight into space weather physics.
We study the initiation and propagation of a CME in the framework of ideal magnetohydrodynamics (MHD). The study comprises a compressible three-dimensional MHD simulation, in which the ideal MHD equations are advanced in time by using an explicit, finite volume solver. We use an idealized model of the solar corona, into which we superimpose a quadrupolar magnetic source region. By applying shearing motions at the solar boundary resembling flux emergence, the initial equilibrium field is energized and it eventually erupts, yielding a fast CME. The simulated CME shows the typical characteristics of a MC as it propagates away from the Sun and interacts with the solar wind. In general, MCs are thought of as large magnetic flux ropes with a considerable amount of twist in it. However, in our simulation 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 a 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 the one described here should also be considered while attempting to explain in-situ observations of regular MCs at L1 and elsewhere in the heliosphere.