Solar Cycle Modulation of Solar Wind Coupling to the AL Index

Abstract:

Coupling strength is defined as the ratio of a system’s output to its input in steady state. The convolution theorem shows that this ratio is given by the area under the system impulse response function. The impulse response can be determined with linear prediction filters. We have developed an optimum coupling function for the AL index and used it to determine the solar cycle dependence of coupling strength. Coupling is weakest at solar maximum (120 nT/mV/m) and strongest at minimum (170). We speculate that this variation is caused by frequent saturation of the polar cap potential at solar maximum. We investigate this hypothesis using local linear filters where data are binned by driver strength. We find for very low driver strength the coupling strength increases with driver strength contrary to expectations. However, above 2 mV/m coupling strength decreases with increasing driver strength as expected from saturation. Averaged over cycle #23 our results show that in the linear regime AL ~212×VBs, and that the saturation electric field is about 12 mV/m. The initial rise in coupling strength for weak driver strength may be a consequence of the westward electrojet moving closer to the AE stations as the polar cap expands. The prediction efficiency of the local filters is never higher than 40% and falls rapidly as driver strength increases. The filters become very difficult to calculate above 10 mV/m.