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

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