Low-altitude field-aligned currents used to diagnose plasma distribution and magnetic topology in the outer magnetosphere

Field-aligned currents (FAC) can be used to determine changes in the total plasma content (TPC) of convecting flux tubes. The observed steady-state FAC system is combined with the observed equipotential pattern to determine contours of TPC as mapped to the ionosphere. Criteria for a qualitative mapping of the FAC, equipotentials and TCP contours along magnetic field lines to the equatorial plane are set up and the result is shown in figure 3. Some interesting features are: (1) There is a considerable distortion, which is most obvious near midnight, due to the existence of FAC; (2) There is a dusk to dawn component of convection across the tail; (3) The reversal of this component in the pre-midnight quadrant produces the Harang discontinuity. A discussion of time-dependent flows suggests that both plasma depletion associated with FAC and neutral lines may be necessary in a substorm expansion. Between substorm expansions, convection is faster than FAC-produced collapse of flux tubes. Finally it is pointed out that the current and electric field are probably not parallel in the tail, requiring a rethinking of tail models.