We propose a description of the substorm expansion phase that includes the formation of a neutral line in the relatively near-earth portion of the tail plasma sheet and phenomena observed in the plasma sheet boundary layer (PSBL). Our proposal is based on previously published observations of the central plasma sheet and PSBL from ISEE-1 at ‖ x ‖<22 Re (CSM coordinates) and of the source region for the PSBL from ISEE-3 near x=-80 Re, as well as on an inferred spatial association of auroral arcs with the PSBL and neutral line signatures observed by other satellites. Specifically, we propose that substorm onset results from the formation of a neutral line within the preexisting source region for the PSBL. The source region is presumably the tail current sheet, which we suggest extends well earthward of 80 Re. Both before and after the neutral line forms, auroral field-aligned currents and large ion flows remain confined to the PSBL earthward of the source region. ¿ American Geophysical Union 1988