Plasma density enhancement and, generation of spatially periodic irregularities in the ionospheric E-region by Powerful HF waves

A new mechamism is investigated to generate spatially periodic temperature acid density irregularities along the geomagnetic field in the ionospheric E-region, using ground transmitted powerful HF (high frequency) waves. A th ermal instability is first excited and then stablized by the nonlinear damp ing from the inelastic electron collisions with N-2 and O-2. Electron tempe rature enhanced by powerful HF waves reduces the recombination rates of ele ctrons and NO+ and O-2(+) ions, and subsequently increases the plasma densi ty. The thermal diffusion of electrons evolves the perturbation into spatia lly periodic density irregularities along the geomagnetic field. The spatia l period between 400 m and 1.6 km of the irregularities varies with die HF wave power and frequency. The optimal conditions for exciting these irregul arities are achievable at sites for ionospheric heating experiments. The in vestigated mechanism works effectively only in the presence of ionization s ources, such as UV radiations during the daytime and precipitated energetic particles at night.