The Two Basic Modes of Magnetospheric Convection Compared

Magnetic flux in the magnetosphere can circulate in a forward sense (southward IMF) and a reverse sense (northward IMF). The point of this presentation is to compare and contrast these two modes of circulation. Both have transient and persistent phases. In the transient phases, reconnection voltage exceeds transpolar potential because for forward circulation there is an inductive 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 magnetospheric erosion is undone by magnetic flux accretion directly onto the dayside magnetosphere with no night-to-day flow of magnetic flux needed. Energy to power the transient and persistent phases of forward circulation comes from the solar wind flow by means of the creation of magnetic stresses as the field is advected tailward. Energy to power the transient phase of reverse convection comes from tapping magnetic energy stored inductively in association with the region 1 and tail current systems. 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. Having exhausted its driving energy source during the transient phase, the persistent phase of reverse circulation is powered 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. We present observations that show a predicted winter-summer asymmetry in polar cap precipitation diagnostic of a rare occurrence of the persistent phase of reverse convection.