We report the double resonance excitation of two pump waves with frequencies separated from 5 to several hundred Hz. The cyclotron frequencies of NO+ and O+2, abundant at the bottom of the F layer (~150 km), are within this range. The nonlinear mode coupling takes place at ~150 km where the pump frequency matches the plasma frequency. The experiments show that when the beat frequency is close to the NO+ cyclotron frequency at ~30 Hz, the energy transfer from the higher frequency to the lower frequency wave is enhanced through parametric coupling. When the two pump waves have equal power, discrete sidebands are symmetrically distributed around the pump frequencies. However, a burst of lower-frequency sidebands, lasting for several seconds, frequently breaks the symmetry. We attribute this asymmetry to the cascade parametric process. The asymmetric spectra only occur during O mode experiments. ¿ American Geophysical Union 1995