The Response of Auroral Electrojets to Steplike Enhancements of Energy Input from the Solar Wind

In this study we examined the response of auroral electrojets to step-like enhancements of energy input from the solar wind to the magnetosphere. As a measure of the intensity of auroral electrojets we used SMU, SML, and SME indices, which are equivalent to the official AU, AL, and AE indices but are derived from a more global data set available through the SuperMAG international collaborative effort (http://SuperMAG.jhuapl.edu). We used the optimal solar wind coupling function proposed by Newell et al. [2007], which we calculated from propagated solar wind and IMF data measured by the ACE spacecraft. This coupling function incorporates the effects of the solar wind velocity, By, Bz, and the IMF clock angle. The best correlation between the coupling function and the SME index occurred when the coupling function was sampled at 80 minutes from the step like change and SME was sampled at 120 minutes from the step like change, for a correlation of 0.674. If we integrated the coupling function, the best correlation occurred when the coupling function was integrated up to 95 minutes from the step like change and the SME index was sampled at 110 minutes from the step like change, for a correlation of 0.664. These results imply that there is a time delay of ~30 minutes, and that although similar solar wind conditions may cause different initial developments of auroral electrojets, they generally become similar after around 2 hours and release similar total amounts of energy. We also examined pairs of events with similar time-profiles of the solar wind energy input. Although we found that auroral electrojets very often develop in similar ways, in some events significant differences can be found in both the timing and intensity of electrojet enhancements. We examined the similarities and differences of the paired events in terms of the seasons of events, but we found that the dependence on season, if at all, is very weak. Based on these results we discuss several issues in predicting geomagnetic activity from solar wind/IMF measurements.