The expansive phase of magnetospheric substorms involves the development of auroral loops and surges with particular emphasis on the westward travelling surge which is identified with the western edge of the substorm current wedge. We use as a working hypothesis the contention that the wavelike auroral structures associated with the current wedge are a manifestation of the action of a Kelvin-Helmholtz instability at the interface between the central plasma sheet (CPS) and the adjacent boundary layer plasmas in the deep magnetotail. Based on this concept, we used ground-based magnetometer data in an attempt to identify the growth of ionospheric current systems which might be associated with the growth of the Kelvin--Helmholtz instability. We have, in fact, found that the expansive phase is preceded by the growth of a weak substorm current wedge which strengthens explosively at the time of onset. We suggest that field-aligned current flows out of the ionosphere as part of a process damping a growing wave at the CPS/boundary layer interface. When the field-aligned current density exceeds ~1 &mgr;A/m2, an acceleration region at altitudes of ~1 RE is activated marking the onset of the substorm expansive phase. ¿American Geophysical Union 1987