Time series of the AL index and rectified solar wind electric field are used to study solar wind coupling to auroral ionospheric currents during individual substorms. Calculation of the individual filters is accomplished by representing them as the product of two simple filters: a low-pass filter that accounts for the response of the ionospheric electric field to the reconnection electric field, and a filter composed of delta functions. The delta functions each have the effect of delaying and scaling the response of AL relative to VBs without distorting the waveform of VBs. Nearly all significant correlation between AL and VBs can be accounted for when the second filter contains only two components. This result suggests that the westward electrojet indexed by AL is controlled by two distinct processes, both proportional to VBs. For an ensemble of 117 isolated substorms the average prediction efficiency of a filter with two delays is 71%. If we were able to determine the exact values of the two delays and the two gain factors prior to an event, then we would be able to estimate the instantaneous magnetospheric response function. We examined season, universal time, and prior activity as possible factors controlling the magnitude of the parameters but found no apparent relationships. Until such relations are found the average parameters provide the best estimate but account for only 47% of the variance in the AL index. ¿ American Geophysical Union 1995.