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 i nstability in the E region of the polar ionosphere to introduce a significa nt electron temperature perturbation. This instability has a broad spatial spectrum with an upper bound. The temperature perturbation then develops no nlinearly, via the electron thermal diffusion process, into spatially perio dic irregularities along the geomagnetic field, where the spatial period of the irregularities in the range between 460 m and 1.3 km is mainly governe d by the minimum wavelength in the spectrum of the instability; Due to the decreasing dependence on the electron temperature of the recombination rate s of electrons with the E region dominant ion;species NO+ and O-2(+), the b ackground plasma density as well as the electrojet current are also perturb ed spatially in a similar fashion as the electron temperature irregularitie s. If an amplitude-modulated HF wave with a modulation frequency in the fre quency range between 2 and 30 kHz is used to modulate the electrojet curren t in time,the density irregularities can effectively convert the current pe rturbation into a spatially distributed mode current of whistler waves. Thi s direct excitation process enhances the generation efficiency of whistler waves and reduces their harmonic components. (C) 2001 American Institute of Physics. [DOI: 10.1063/1.1334610].