During recent experiments at Arecibo, Puerto Rico, large (1000--2000 K) enhancements in electron temperature have been observed when high-power radio waves reflect near the nighttime F region peak. The electron temperature enhancements are accompanied by significant (50--300 K) increases in ion temperature, large (10--15%) reductions in electron density, and strong HF-induced spread F. When large electron temperature enhancements develop, the ionosphere appears to become dynamically unstable resulting in the production of geomagnetic field-aligned striations having elevated electron temperatures and depleted electron densities. ¿ American Geophysical Union 1987