The nonlinear response of AE to the IMF BS driver: A spectral break at 5 hours

The authors demonstrate the existence of a sharp break in the power spectrum of AE at {approximately}5 hours. At frequencies below the break, AE has a {approximately}f{sup {minus}1.0} dependence, and at higher frequencies it has a f{sup {minus}2.2} to f{sup {minus}2.4} dependence. The power spectrum of the IMF B{sub s} for the same time interval has a {approximately}f{sup {minus}1.4} dependence with no spectral breaks or peaks. Thus, at frequencies above the break, the power in AE varies as f{sup {minus}0.5} to f{sup {minus}0.8} times the power in B{sub s} and at low frequencies it ranges from being f{sup 0} (frequency independent) to a f{sup +0.4} times the power in B{sub s}. Several possible explanations of the nonlinear response of AE to the IMF B{sub s} driver are briefly discussed, including: (1) variable ionospheric conductivity (increasing with B{sub s}) for the high frequency regime, and (2) several AE saturation mechanisms for the low frequency regime.