Modification of the ionosphere by means of high power radio waves.

Experiments have been performed at Troms?, Norway and Arecibo, Puerto Rico to investigate the effects produced by high power radio waves (heating) on the high and low latitude ionospheres, respectively. The modifications induced in the F-region have been observed with both incoherent scatter radars and tow power HF diagnostic radio waves. These observations provide information on both heater-induced large scale changes, and small scale density structures. The spatial and temporal development of the perturbation in electron temperature caused by heating, measured at Troms? for the first time, compares well with theory. Observations of thermal striations, which may be the result of the thermal self-focussing of the heater wave, are also presented. The large-scale heater-induced changes in the electron density measured by EISCAT have been compared with those derived from measurements of the phase of reflected HF diagnostic signals. Both methods yield consistent results with regard to whether the electron density is enhanced or depleted but there are, however, differences in the magnitudes of the measured changes. The sign of the perturbation can reveal aeronomical information on the chemical and diffusion characteristics of the unheated ionosphere. The first observations of tow latitude heating by means of low power HF diagnostic waves are presented. The anomalous absorption measured at tow latitudes is less than that at high latitudes, in agreement with theory. The scale length and amplitude of the associated small-scale density structures are comparable at both high and low latitudes. A detailed comparison has been undertaken of the effect of heating at high and low latitudes and the results compared with current theoretical models. The two independent diagnostic techniques (incoherent scatter and low power HF waves) provide a consistent picture of the ionospheric perturbation induced by both heating facilities.