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 BS 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.05 to f-0.8 times the power in BS and at low frequencies it ranges from being f0 (frequency independent) to a f+0.4 times the power in BS. Several possible explanations of the nonlinear response of AE to the IMF BS driver ar briefly discussed, including: 1) variable ionospheric conductivity (increasing with BS) for the high frequency regime, and 2) several AE saturation mechanisms for the low frequency regime. ¿ American Geophysical Union 1990