Comparison between the two basic modes of magnetospheric convection

摘要

Magnetic flux in the magnetosphere can circulate in a forward sense (southward IMF) and a reverse sense (northward IMF). The point of this paper is to compare and contrast these two modes of circulation. Both have transient and persistent phases in response to a suddenly applied southward or northward IMF that then is held steady. In the initial, transient phases, reconnection voltage exceeds transpolar potential in the ionosphere because for forward circulation there is an inductive-like time lag while the region 1 current builds up in association with a net flow of magnetic flux from the dayside to the tail (magnetospheric erosion). For the transient phase of reverse convection, reconnection voltage exceeds transpolar potential because instead of over-the-pole, night-to-day transport to undo preexisting magnetospheric erosion, magnetic flux accretes directly onto the dayside magnetosphere. The persistent phase of forward circulation almost always manifests substorms, which modulate the amounts of open and closed flux on the nightside by about 25% in a zero-sum game that leaves the dayside flux mostly unaltered. Energy to power the transient and persistent phases of forward circulation comes from the solar wind flow through magnetic stresses created as the field advects tailward. Energy to power the transient phase of reverse convection, however, comes from tapping magnetic energy stored inductively in association with the region 1 and tail current systems. The persistent phase of reverse circulation is driven possibly by the Song-Russell mechanism, which operates on a day-to-night pressure gradient of plasma entrained by dayside flux accretion. The resulting circulation is slow.