Near upper hybrid resonance an O mode launched from a ground-based transmitter undergoes two possible parametric decay instabilities: (1) a resonance instability producing field-aligned dc density striations and an upper hybrid wave and (2) a parametric decay instability producing lower hybrid and electron Bernstein modes. A nonlinear coupling between the lower and upper hybrid waves produces O mode sideband radiation. The lower sideband is severely cyclotron damped, while the upper sideband is weakly damped and hence observed on the ground. This explains some of the experimental results reported from the Tromso facility. ¿ American Geophysical Union 1993