Recent Work on Modelling the Global Field Line Topology of Interplanetary Magnetic Clouds

Abstract

Magnetic clouds form a distinct and important subset of coronal mass ejections (CME's) distinguished by a few simple properties. In recent years there has been lively interest in modelling the global magnetic field line topology of magnetic clouds. Are they closed magnetic configurations disconnected from the Sun (magnetic bubbles), or are they better described as magnetic loops or tongues or flux ropes (magnetic bottles), perhaps even retaining magnetic connection to the Sun ? In this selective review we discuss the arguments advanced in favour of one or other of two models developed enough to allow meaningful comparison of theoretical predictions with in situ spacecraft data: on the one hand, a magnetic flux rope and, on the other, a spheromak. The experimental datum that magnetic clouds are strongly evolving configurations will be central to the discussion. Our comparisons will be based on exact solutions of the ideal MHD equations describing the radial, self-similar expansion of cylindrical magnetic flux ropes and spheromaks. On present evidence, this comparison favours the magnetic flux rope topology. We shall also briefly discuss recent MHD simulations of the propagation of magnetic clouds into interplanetary space and how the results thus obtained compare with analytical conclusions. A short discussion of magnetic field line draping as a potential means of distinguishing the two topologies follows. Finally, we point out the importance in further analytical elaboration of the models of incorporating the interaction of magnetic clouds with surrounding flows.