| Summary: | A high power radio wave propagating through the ionosphere perturbs the natural plasma in a number of ways. These include bulk changes in the electron temperature and density, and the excitation of plasma waves and instabilities. The ionosphere can be regarded as a natural plasma laboratory, with far greater scale sizes then anything achievable in a conventional laboratory, thus giving unique access to a range of plasma waves and instabilities. In this thesis results from two campaigns involving the EISCAT (European Incoherent SCATter) ionospheric modification facility (heater) at Tromso, Northern Norway are presented.;The first campaign, in February 1995, was principally concerned with modification of the auroral electrojet region during both disturbed and quiet conditions. During this campaign the EISCAT UHF incoherent scatter radar was utilised to achieve the first experimental demonstration of artificial inospheric cooling by a high power radio wave.;For the second campaign, in April 1996, the most important diagnostic was CUTLASS (Co-operative UK Twin Located Auroral Sounding System), which can measure power backscattered from field aligned irregularities produced during heating. It has been found that irregularities are excited over a horizontal spatial extent of around 175 km, which far exceeds the normal width of the heater beam. The temporal decay of artificial irregularities remain in the ionosphere for up to four minutes after heater-off.;In addition to these experimental studies, a computer model has been developed to investigate the electron temperature and density changes which are induced in the ionospheric D- and E-regions during periods of electrojet activity. The model reproduces the experimental observations, and predicts reduced artificial heating efficiency under distributed natural conditions.
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