A new mechanism is investigated to generate spatially periodic temperature and density irregularities along the geomagnetic field in the ionospheric E-region, using ground transmitted powerful HF (high frequency) waves. A thermal instability is first excited and then stabilized by the nonlinear damping from the inelastic electron collisions with N2 and O2. Electron temperature enhanced by powerful HF waves reduces the recombination rates of electrons and NO+ and O2+ ions, and subsequently increases the plasma density. The thermal diffusion of electrons evolves the perturbation into spatially periodic density irregularities along the geomagnetic field. The spatial period between 400 m and 1.6 km of the irregularities varies with the HF wave power and frequency. The optimal conditions for exciting these irregularities are achievable at sites for ionospheric heating experiments. The investigated mechanism works effectively only in the presence of ionization sources, such as UV radiations during the daytime and precipitated energetic particles at night. ¿ 1999 American Geophysical Union