Throughout most of the growth phase of a substorm, the cross-tail current at x~-10 Re can be supplied by the curvature drift of a bi-directional field aligned distribution of ~1 keV electrons. Just prior to its local disruption after substorm onset, the cross-tail current in the now thin (~400 km) current sheet is carried by the cross-tail serpentine motion of non-adiabatic ions (Speiser, 1965). The instability of this latter current leads to the local disruption of the near-earth current sheet. ¿ American Geophysical Union 1990