We use SuperDARN radar measurements of ionospheric plasma flow on the dayside to evaluate the extent to which substorm onset is the result of interplanetary magnetic field (IMF) changes that lead to a reduction in the strength of magnetospheric convection. We also use this data to evaluate the timing of such convection changes relative to the time of onset. By using direct observations of convection, we avoid the uncertainties in previous studies of substorm triggering arising from their use of IMF measurements from locations away from the magnetosphere. We first identified 12 well-defined substorm onsets for which sufficient SuperDARN echo coverage on the dayside was available for our analysis. We find that at least 10 of the onsets were associated with a reduction in the strength of large-scale convection that is imparted to the magnetosphere from the solar wind. This indicates that the majority of substorms with well-defined onsets are associated with such a convection reduction. We also find that the convection reduction imparted to the magnetosphere initiates a few minutes prior to substorm onset. This is the same time relative to onset that based on auroral observations, the nightside magnetosphere undergoes a transition to an unstable configuration. This suggests that the convection reduction is responsible for the transition to instability that leads to the substorm expansion phase. Our results are consistent with the idea that under enhanced magnetospheric convection, energy builds up within the nightside magnetosphere and can reach a stable equilibrium where energy losses balance further energy input. If the energy accumulated on the nightside during a period of enhanced convection exceeds that of the equilibrium configuration after a reduction in the strength of convection, the nightside magnetosphere undergoes a transition to instability when the strength of convection is reduced and the excess energy is released as the substorm expansion phase. These results indicate that the question of how this instability arises and develops after a decrease in convection is a critical outstanding question for understanding the substorm expansion phase.