Using amplitude-modulated high-frequency (HF) heating waves for electron heating, the conductivity of plasma and thus the embedded electrojet currents in high-latitude ionosphere can be modulated accordingly to set up the ionospheric antenna current for extremely low frequency (ELF) wave generation. Electron heating is hampered by inelastic collisions of electrons with neutral particles (mainly due to vibration excitation of N2), which cause the induced modulation current to remain at a low level. However, this inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. Thus, as the heating power exceeds a critical level, electron heating suddenly reaches an unexpected high level, resulting to a near step increase (of about 5 to 8 dB depending on the modulation waveform and frequency) in the spectral intensity of ELF radiation. The dependency of this critical HF heating-wave power on the modulation frequency is determined for three heating wave modulation forms: (1) rectangular wave, (2) sine wave, and (3) half-wave rectified wave.