EISCAT OBSERVATIONS OF ELECTRON-TEMPERATURE OSCILLATIONS DUE TO THE ACTION OF HIGH-POWER HF RADIO-WAVES

Heater induced electron temperature and electron density temporal fluc tuations have been observed for the first time in EISCAT backscatter d ata at Tromso. These fluctuations are also accompanied by large scale (bulk) changes in electron temperature and electron density. The maxim um heating effect is observed near 190 km where photochemical processe s are important, in contrast to most published data where heating is r eported to take place at higher altitudes where transport processes ar e dominant. Data from two separate experiments on 19 and 26 February 1 987 are presented. In both experiments these fluctuations are observed throughout the F-region during full power overdense heating, with the maximum fluctuation amplitude being located at the heater reflection height. In an experiment on 19 February 1987, the maximum amplitudes o f the fluctuations in the electron temperature and the electron densit y are comparable. However, in the experiment on 26 February 1987, ther e is a considerable natural variation in the ambient electron density which makes it impossible to distinguish between the natural and heate r induced changes in electron density and only fluctuations in electro n temperature are clearly observable. The data presented in this paper indicate that the coupled electron temperature and electron density o scillation is driven by chemical perturbation rather than the self foc using instability found in transport dominated regions. Spectral analy sis of the data indicates that the strongest fluctuations have periods of 2-5 min with scale sizes perpendicular to the magnetic field of be tween 6 and 10 km.