Two radio waves of frequencies &ohgr;1 and &ohgr;2 couple nonlinearly in the lower ionosphere to create a temperature perturbation TΔ&ohgr;,2k1 (Δ&ohgr;=&ohgr;1-&ohgr;2; 2k1≈k1+k2) and a density perturbation nΔ&ohgr; by the process of ambipolar diffusion. This perturbation couples with the &ohgr;1 frequency wave to generate electromagnetic sidebands of frequencies &ohgr;1¿nΔ&ohgr; (n=1, 2, ... ) as observed in a recent experiment. The field of the sideband wave near the turning point has an Airy function dependence while away from the turning point it has a WKB sinusoidal solution. The power of the sideband decreases with increasing Δ&ohgr;. It also falls almost exponentially with the order of the sideband. The existence of a turning point for the pump waves is essential for efficient nonlinear coupling. ¿ American Geophysical Union 1988