We have used data from the November 24, 1996, Geospace Environment Modeling program substorm event to investigate the critical question of substorm expansion phase triggering by interplanetary magnetic field (IMF) changes that lead to a reduction in large-scale convection. Coverage of ionospheric convection with the Super Dual Auroral Radar Network (SuperDARN) radars was excellent during this period. This has allowed us to investigate the changes in large-scale convection associated with two expansion phase onsets that followed a 80-min period of strongly southward and quite steady IMF. We have found that reductions in global convection occurred in association with two observed northward turnings of the IMF, and, to within observation uncertainty, these reductions occurred at the same times that the two onsets were first observed in the magnetosphere. The first (smaller) northward turning was found to give a small convection reduction and the onset of a small substorm, while the second (much larger) northward turning was found to give a large reduction in global convection and the onset of a large substorm. Taken together with other, more localized studies which also show convection reductions associated with expansion onsets, our results imply that IMF-driven reductions in global convection may be generally associated with expansion onset. Our results also suggest that substorm expansions may represent an adjustment to conditions of reduced convection following a growth phase of enhanced convection and magnetotail energy storage. Under this suggestion, for a given amount of energy stored in the tail, a small reduction in convection would lead to a small substorm, and a large reduction in convection would lead to a large substorm. ¿ 2001 American Geophysical Union