Variability in the response of the magnetosphere under different interplanetary magnetic field (IMF) conditions is investigated by utilizing a high-resolution, three-dimensional, global magnetohydrodynamic (MHD) model. Changes in the MHD model response time delay are found by performing a parameter search based on the strength of the IMF Bz component, which is held constant for 4 hours prior to and following southward turnings. The parameter search reveals that variations in the growth phase duration and, thus in the timing of expansion phase onset, follow a simple pattern governed by the relative strengths of IMF Bz before and after the southward turnings. If the IMF northward and southward values are equal in magnitude, the expansion phase onset occurs ~60 min after dayside merging begins. However, if the north IMF setting is smaller (larger) than the subsequent southward field intensity, the expansion phase onset can occur after a much shorter (longer) time delay. The range of onset delay times found from the simulation is ~60¿30 min. The MHD results are tested by means of an empirical analysis, which is based on the assumption that the solar wind response of the magnetosphere, as represented by the AL index, is bimodal, owing to the different solar wind response timescales of the convection current system and the substorm current wedge. The empirical analysis utilizes short duration time intervals including solar wind input time series and corresponding AL index data. The empirical results agree favorably with the results found via MHD simulation. ¿ 1999 American Geophysical Union