A numerical study of the dynamics of the plasma tail of comets

We study numerically the dynamics of the plasma tail with a super-Alfvénic flow in the lobes on the basis of a compressible MHD model. The results show that flow tearing mode instability is excited by the sheared flow which exists between the plasma sheet and the lobes. It causes field reconnection and forms magnetic islands and high-density plasmoids that move tailward. The effect of a local driving force due to the solar wind at the boundary is also simulated, but its characteristic time is found to be much larger than that of the flow tearing mode instability. It is suggested that some of the knots and disconnection events observed in the plasma tails of comets are associated with flow tearing mode instability.