Since substorm expansive phase activity in the midnight sector and Pc 4,5 pulsation activity in the morning sector are both colocated with the auroral oval, it is logical to assume that sudden changes in plasma and field parameters in the oval would cause perturbations in the activity level of both types of activity. In an earlier study, Samson and Rostoker [1981> demonstrated that the frequency of Pc 4,5 pulsations on the dayside increased at the onset of a substorm expansive phase in the night sector. In that study, only pulsations recorded near noon were investigated for their frequency content. In this paper we consider the response of pulsation activity across the entire morning sector to substorm onsets near midnight. Using superposed epoch analyses we demonstrate that the conclusions of Samson and Rostoker [1981> are valid across the morning sector. However close to dawn, Pc 5 activity appears to be initiated at substorm onset rather than decreasing by shifting frequency toward the Pc 5 band, as appears to be the case closer to noon. We present a model of Pc 4,5 activity to explain this phenomenon, in which the origin of the pulsations is proposed to be a Kelvin-Helmholtz instability at the interface between the low-latitude boundary layer and the central plasma sheet.