Artificial ionospheric modification experiments by means of high power, high frequency radio waves using the EISCAT (European Incoherent SCATter) heater at Tromsø, northern Norway have been carried out for nearly three decades. These experiments are associated with large-scale increases in the electron temperature and density and generation of small-scale plasma irregularities, which are diagnosed by the EISCAT UHF and CUTLASS (Cooperative UK Twin Located Auroral Sounding System) HF radars.\ud Experimental results from the EISCAT UHF radar, obtained from heating campaigns at\ud Tromsø between 1996 and 1999, are used to study the effects of heater-induced instabilities on the analyses of EISCAT spectra. Plasma turbulence induced by high power heater waves produces unphysical estimates of ionospheric plasma parameters during the initial phase of heating, and a method is developed to correct for these problems. It is shown that these estimates are nevertheless, reliable over longer timescales of heating.\ud In the second study, relaxation characteristics of small-scale density perturbations measured using the CUTLASS Finland radar are modelled theoretically and shown to be largely consistent with large-scale temperatures measured using the EISCAT UHF radar. Certain cases are identified where alternative models of irregularity decay are required to describe the temporal behaviour in radar backscatter. These models consider influences such as electron temperature reductions, wave-wave coupling and background flow.\ud The final study concentrates on a heating experiment in October 2000, where unusually broadened spectral widths of artificial HF backscatter measured with the CUTLASS Finland and Iceland radars was observed. The results were incompatible with previous explanations of similar observations. The Burg maximum entropy analysis was implemented to formulate a tentative explanation for the observations, involving simultaneous detection of artificial and natural irregularity echoes by the Iceland radar.\ud This required the heater amplification of non co-located natural irregularities, already above the noise threshold of the radar
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