Generation of ELF waves by stimulated parametric coupling of two HF waves in the lower ionosphere is considered. In this region the nonlinear force is dominated by the thermal rather than the ponderomotive nonlinearity. It is shown that this results in lowering the pump threshold for complete decay by more than an order of magnitude while achieving efficiencies in excess of those expected on the basis of the Manley-Rowe relations. The lower-frequency mode excited for E region altitudes is the helicon mode, which continues to frequencies below the ion cyclotron frequency because ion-neutral collisions freeze the ion motion. The application of these results to ELF generation in the lower ionosphere, including power estimates for a proof of principle experiment, is discussed.