Heikkila's mechanism for impulsive plasma transport through the magnetopause: A reexamination

Abstract

It has been proposed by Heikkila (1982) that the impact upon a magnetically “open” magnetopause current sheet of a “cloud” of magnetosheath plasma having excess momentum will result in the transport of the cloud through the sheet, forming a plasma boundary layer located on both open and closed flux tubes in the interior. A reexamination of Heikkila's work shows, however, that the argument which was used to arrive at that conclusion is not correct. The error arises from the assumption that the induction electric field which is associated with the perturbation of the current sheet will produce a flow which is just such as to cause the plasma to move with the sheet. This is not the case. The flow normal to the current sheet which is produced by the induction electric field alone will in general be less than the speed of the sheet itself, continuity being maintained by flows directed along the perturbed current layer. If we then assume, with Heikkila, that the induction electric field component which is parallel to the magnetic field component which threads the “open” magnetopause is cancelled to zero by a curl‐free charge‐separation electric field, then the tangential electric field and normal flow are indeed increased, as argued by Heikkila, but only to the point where the flow just matches the speed of the boundary, and not to a speed which is in excess of the latter, as claimed. Heikkila's mechanism does not therefore lead to “impulsive transport” of magnetosheath plasma through the boundary onto open and closed magnetospheric flux tubes in the interior. These statements concerning the relationship between the plasma flow and the motion of the boundary refer explicitly to a formulation of the problem, consistent with Heikkila's discussion, in which (1) E.B is taken to be zero everywhere, (2) the reconnection‐associated electric field is zero (or is transformed to zero by a deHoffman‐Teller transformation), (3) the plasma motion is taken to be the sum of a field‐aligned motion and E×B drift, and (4) the plasma flow streamlines are taken to lie in planes transverse to the magnetic field component which reverses across the current sheet. However, plasma motions directed along the magnetic field can be added arbitrarily to these flows, so that plasma can indeed cross the boundary in either direction, but only by flow along the open field lines. Consequently, magnetosheath plasma can indeed flow through the open magnetopause to form a boundary layer inside the magnetosphere, but this layer will be confined to open flux tubes adjacent to the boundary as in reconnection models. Boundary layers are not formed on adjacent closed flux tubes by these means. Our purpose here is not to dispute the experimental evidence which indicates that boundary layers are sometimes formed on closed flux tubes, but only to point out that Heikkila's argument does not provide a valid mechanism which leads to their formation.