Abstract

Powerful radio waves can heat an electron gas via collisions between free electrons and neutral particles. Since the discovery of the Luxembourg effect in 1934, this effect is known to take place in the D-region ionosphere. According to theoretical models, the EISCAT Heating facility is capable of increasing the electron temperature by a factor of 5–10 in the D region, depending mostly on the electron density profile. Various indirect evidence for the existence of the D-region heating effect has been available, including successful modification of ionospheric conductivities and mesospheric chemistry. However, an experimental quantification of the electron temperature at its maximum in the heated D-region ionosphere has been missing. In particular, incoherent scatter (IS) radars should be able to observe directly plasma parameters, such as the electron temperature, although the heated D-region ionosphere is not a trivial target because of low electron density, and hence, small signal-to-noise ratio (SNR).

In this thesis, Papers I and III present unique estimates for heated D-region electron temperatures based on IS measurements. It turned out that the theoretical predictions of the electron temperature generally agree with the few existing observations, at least at the altitudes of the maximum heating effect.

Quite in contrast, when the D-region heating effect on the cosmic radio noise absorption was verified for the first time by the statistical data analysis presented in Paper II, the absorption enhancements due to heating were found to be an order of magnitude smaller than model results. The reason for this discrepancy remains still as open question, although one possible explanation is provided by the electron-temperature dependent ion chemistry, which was not taken into account in the modelling. The significance of the heating-induced ion chemistry effect in the D-region was investigated in Paper IV. There the heating-induced negative ion formation is proposed as a potential explanation for the observed modulation of Polar Mesosphere Winter Echo (PMWE) power.