Two fortuitous polar passes of the Isis 2 satellite are used to examine the location of auroral arcs near substorm onsets in relation to the low-altitude magnetospheric plasma boundaries. In one case the auroral arc was brightening at substorm onset during the pass. In the other case the satellite passed the arc after its brightening but just prior to its poleward expansion. In both cases the arcs are situated near the transition between the central plasma layer and the boundary plasma layer as defined by Winningham et al. [1975>. The arc location was also near the poleward termination of the durably trapped energetic electrons (E>40 keV). Thus these observations suggest that the substorm triggering process occurs on closed magnetic field lines well within the outer and inner edges of the plasma sheet.