ANALYSIS OF THE LINEAR-RESPONSE FUNCTION RELATING AL TO VBS FOR INDIVIDUAL SUBSTORMS

Time series of the AL index and rectified solar wind electric field ar e used to study solar wind coupling to auroral ionospheric currents du ring individual substorms. Calculation of the individual filters is ac complished by representing them as the product of two simple filters: a low-pass filter that accounts for the response of the ionospheric el ectric field to the reconnection electric field, and a filter composed of delta functions. The delta functions each have the effect of delay ing 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 com ponents. 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 effic iency pf 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 magnetos pheric response function. We examined season, universal time, and prio r activity as possible factors controlling the magnitude of the parame ters but found no apparent relationships. Until such relations are fou nd the average parameters provide the best estimate but account for on ly 47% of the variance in the AL index.