This is the region where the earthward plasma flow from the more distant plasma sheet is broken and is diverted northward and southward by the dipole field. Examination of panels a and e of Figures 4 - 7 suggest that the dipolarization is due to flow diversion. This is consistent with the proposal of Kan [1998] that a necessary condition for substorm onset is abrupt convection breaking in the near-Earth plasma sheet. We have also seen that the region behind the dipolarization is characterized by high levels of turbulence and is the likely source of the filamentary currents. Takhashi et al. [1987] have indeed observed high levels of turbulence in the near-Earth plasma sheet. The dipolarization has been associated with disruption of the cross-tail current [Lui, 1999]. While there may be current disruption, the simulations suggest it is incidental to the main events, namely the collision of the tail plasma with the more stationary near-Earth plasma and the generation of the field-aligned currents that may be responsible for the discrete aurora seen in the auroral expansion.

Further earthward, we observe a large pileup of plasma. This effect, as well as the north-south diversion, is likely to be considerably exaggerated in the two-dimensional simulation described here, because there is no east-west direction in which to divert the plasma flow. However, injections of plasma coincident with auroral brightening [Akasofu, et al., 1974] are indeed observed.

Contents

References

Section 5