Magnetohydrodynamic simulations of the motion of magnetic flux tubes through a magnetized plasma

Magnetohydrodynamic simulations of the evolution of a flux tube accelerated through a stationary magnetized plasma are presented. As the flux tube moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fields. The coupling between the moving flux tube and the external plasma is generally efficient, with simulated flux tube velocities many times smaller than those expected from frictionless motion. The reconnection between the flux tube and external field takes place when there is a unidirectional external field component in the direction of flux tube propagation. The reconnection is intrinsically nonsymmetric around the flux tube boundary. The principal reconnection site is at the rear of the flux tube, where strong vortices convect the external field toward the flux tube. Drag coefficients (CD) that parameterize this interaction have been determined. When the flux tube is continually accelerated, CD > 1 is appropriate, consistent with previously used ad hoc values. Examples of when the flux tube is accelerated for a short time but allowed to continue interacting with the external plasma are presented. It is shown that in the absence of reconnection, the coupling time is several Alfvén wave transit times across the flux tube. However, when reconnection takes place, this coupling can cease to occur, and the flux tube may move frictionlessly (CD ≈ O). The results are discussed in terms of interplanetary magnetic clouds, and it is suggested that the observations of comoving coronal mass ejection and solar wind plasma can be accounted for by drag between the two.