Generation of density irregularities and whistler waves by powerful radio waves in the polar ionosphere

It is shown that a powerful high-frequency (HF) wave can excite a thermal instability in the E region of the polar ionosphere to introduce a significant electron temperature perturbation. This instability has a broad spatial spectrum with an upper bound. The temperature perturbation then develops nonlinearly, via the electron thermal diffusion process, into spatially periodic irregularities along the geomagnetic field, where the spatial period of the irregularities in the range between 460 m and 1.3 km is mainly governed by the minimum wavelength in the spectrum of the instability. Due to the decreasing dependence on the electron temperature of the recombination rates of electrons with the E region dominant ion species NO⁺ and O₂⁺, the background plasma density as well as the electrojet current are also perturbed spatially in a similar fashion as the electron temperature irregularities. If an amplitude-modulated HF wave with a modulation frequency in the frequency range between 2 and 30 kHz. is used to modulate the electrojet current in time, the density irregularities can effectively convert the current perturbation into a spatially distributed mode current of whistler waves. This direct excitation process enhances the generation efficiency of whistler waves and reduces their harmonic components.