Downstream structure and evolution of a simulated CME-driven sheath in the solar corona

Context. The transition of the magnetic field from the ambient magnetic field to the ejecta in the sheath downstream of a coronal mass ejection (CME) driven shock is analyzed in detail. The field rotation in the sheath occurs in a two-layer structure. In the first layer, layer 1, the magnetic field rotates in the coplanarity plane (plane of shock normal and the upstream magnetic field), and in layer 2 rotates off this plane. We investigate the evolution of the two layers as the sheath evolves away from the Sun. Aims. In situ observations have shown that the magnetic field in the sheath region in front of an interplanetary coronal mass ejection (ICME) form a planar magnetic structure, and the magnetic field lines drape around the flux tube. Our objective is to investigate the magnetic configuration of the CME near the sun. Methods. We used the space weather modeling framework (SWMF), a 3D magnetohydrodynamics (MHD) simulation code, to simulate the propagation of CMEs and the shock driven by it. Results. We find that close to the Sun, layer 2 dominates the width of the sheath, diminishing its importance as the sheath evolves away from the Sun, in agreement with observations at 1 AU.