Magnetic structure of overexpanding coronal mass ejections: Numerical models

Numerical simulations are presented of the evolution of overexpanding coronal mass ejections (OCMEs), which are also magnetic clouds. The OCME is assumed to arise from the evolution of a magnetic flux rope with high plasma and magnetic pressure and high plasma density near the Sun in a high-speed solar wind. It is shown that the flux rope maintains its integrity from near the Sun to approximate to 5 AU, resisting hydrodynamic forces that tend to distort it. Thus OCMEs that are magnetic clouds at large heliocentric distances should have simply evolved from near-Sun flux ropes. It is shown that an initially circular flux rope is distorted into a hemispheric shape by its interaction with solar wind plasma hows. Forward and reverse shock pairs form with the forward shock being curved while the reverse shock is straight. The magnetic field properties at large distances are shown to depend on whether the initial flux rope undergoes overexpansion. A flux rope that is convected passively in the solar wind without overexpansion will ultimately have a magnetic field profile dominated by its toroidal component, so would not be observed as a magnetic cloud. The overexpanding flux ropes modeled here maintain an approximately equal ratio of toroidal to poloidal magnetic fields. The relative initial speed of the flux rope with respect to the solar wind does not influence the large-scale magnetic properties up to 5 AU, although it does affect the detailed field topology.