Generation of ELF and VLF waves by a thermal instability excited in the HF heater‐modulated polar electrojet

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.