MAGNETOHYDRODYNAMIC SIMULATIONS OF THE MOTION OF MAGNETIC-FLUX TUBES THROUGH A MAGNETIZED PLASMA

Magnetohydrodynamic simulations of the evolution of a flux tube accele rated through a stationary magnetized plasma are presented. As the flu x tube. moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fi elds. The coupling between the moving flux tube and the external plasm a is generally efficient, with simulated flux tube velocities many tim es smaller than those expected from frictionless motion. The reconnect ion between the flux tube and external field takes place when there is a unidirectional external field component in the direction of flux tu be 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 to ward the flux tube. Drag coefficients (C-D) that parameterize this int eraction have been determined. When the flux tube is continually accel erated, C-D > 1 is appropriate, consistent with previously used ad hoc values. Examples of when the flux tube is accelerated for a short tim e but allowed to continue interacting with the external plasma are pre sented. It is shown that in the absence of reconnection, the coupling time is several Alfven wave transit times across the flux tube. Howeve r, when reconnection takes place, this coupling can cease to occur, an d the flux tube may move frictionlessly (C-D approximate to 0). The re sults 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.