A thermal instability responding to the modulation of polar electrojet by HF heater as a potential efficient mechanism for the generation of ELF and VLF waves has been investigated. It is shown that a positive feedback through the electron-neutral collisional heating process can cause the transient response of the plasma to the modulated HF heater to grow exponentially. Considering a sinusoidally modulated HF heater wave having its field amplitude proportional to ‖cosω1t/2‖, the threshold fields of the thermal instability under normal electrojet conditions are found to be about 1.6 V m-1 and 0.8 V m-1 for the o mode and x mode operation respectively. For a heater wave field of 2 V m-1, the instability can be excited by the x mode heater within a few milliseconds and also by the o mode heater but taking about 9 times as long. ¿ American Geophysical Union 1993