Decoupling of Convection in the Magnetosphere From the Ionosphere by Parallel Electric Fields

Field-aligned potential drops (electric fields) result in different patterns of equipotentials of the electric fields in the outer magnetosphere and in the ionosphere. Thus, over limited areas, magnetospheric convection can proceed without the line-tying effect of the ionosphere. Such convection is faster than when it is coupled to the ionosphere. Diagrams can be drawn which illustrate possible configurations of equipotentials for decoupling on a set of field lines carrying the upward field-aligned current. Field-aligned potential drops have been observed to be more than 10 kV in substorms, particularly at the leading edge of the westward surge. Thus it seems likely that significant rapid convection, decoupled from the ionosphere, occurs in the earthward direction in the outer magnetosphere at these times. Both the sudden onset of a field-aligned potential drop and the formation of an X line offer possible explanations of the sudden onset of substorm expansions. It is likely that one causes the other since both are observed to occur in substorms. This cause-effect relationship is probably best determined from observations.