High power electromagnetic waves are able to modify the Earth’s ionosphere. A number of plasma waves and instabilities may be excited in this way, including patches of small-scale field-aligned electron density irregularities. The EISCAT heater, situated in northern Norway, has been used to transmit high power, high frequency radio waves into the ionosphere since its construction in the early 1980s. The CUTLASS coherent backscatter radars have been able to measure the consequent electron density perturbations in the ionosphere above the heater since their construction in the mid 1990s. This thesis contains a statistical study of CUTLASS Finland backscatter power measurements of patches of irregularities excited by the EISCAT heater during campaigns carried out between 1996 and 2002.\ud A study of CUTLASS backscatter power measurements made during heater transmissions carried out at a fixed heater beam pointing direction, frequency and power has provided fundamental information on the effect of these heater beam parameters on the irregularities.\ud A study of a selection of backscatter data from above the SPEAR heating facility has shown similar results.\ud Backscatter data corresponding to experiments involving heater beam-sweeping and power-stepping have also been analysed. During beam-sweeping experiments the heater beam direction is steadily changed from northwards- to southwards-pointing. During power-stepping experiments the heater beam power is steadily increased and decreased. The results from these experiments give compelling evidence in support of the upper hybrid theory of irregularity excitation.\ud Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modelled heater beam power distributions provided by the EZNEC4 software. In general a good agreement between the statistical parameters is observed, indicating the large influence of the heater beam on the irregularities. The influence of the Earth’s magnetic field is also evident and often leads to discrepancies between the statistical parameters of the two sets of distributions
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