EISCAT observations of electron temperature oscillations due to the action of high power HF radio waves

Heater induced electron temperature and electron density temporal fluctuations have been observed for the first time in EISCAT backscatter data at Tromsø. These fluctuations are also accompanied by large scale (bulk) changes in electron temperature and electron density. The maximum heating effect is observed near 190 km where photochemical processes are important, in contrast to most published data where heating is reported to take place at higher altitudes where transport processes are dominant. Data from two separate experiments on 19 and 26 February 1987 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 of the fluctuations in the electron temperature and the electron density arc comparable. However, in the experiment on 26 February 1987, there is a considerable natural variation in the ambient electron density which makes it impossible to distinguish between the natural and heater induced changes in electron density and only fluctuations in electron temperature are clearly observable. The data presented in this paper indicate that the coupled electron temperature and electron density oscillation is driven by chemical perturbation rather than the self focusing instability found in transport dominated regions. Spectral analysis of the data indicates that the strongest fluctuations have periods of 2–5 min with scale sizes perpendicular to the magnetic field of between 6 and 10 km.