| Summary: | In order to characterize the features of the Ionospheric Alfvén Resonator (IAR) at high latitudes in both northern and southern hemispheres, both data interpretation and numerical computation are presented. Four IAR events observed by the pulsation magnetometer at Sodankylä in a single month were statistically analysed. It was found that the IAR eigenfrequency separations fluctuate with time. The fluctuation was dominated by plasma density perturbation in the ionosphere. Also, a single IAR event observed by five pulsation magnetometers simultaneously was analysed. The analysis showed the eigenfrequencies of the single IAR detected at different locations are different. Additionally, the eigenfrequency shifts were found to differ at different locations. It indicates that the horizontal scale of a single IAR event could be up to thousands of kilometres. The horizontal structure of the IAR in a large scale is non-uniform. Also, the study has revealed that the visibility of the Spectrum Resonance Structure (SRS) strongly depends on the fluctuation rate of the eigenfrequency separations. Moreover, the first study of IARs in Antarctica was carried out. The IAR occurrence and the relation with the solar activities were investigated statistically. In addition, a numerical model was introduced in this thesis. Based on this model the boundary condition dependence of the IAR was investigated. According to the study the detected eigenfrequencies, the spatial structures of the field and the ratio between the intensities of the total current and the source current strongly depend on the ratio between the wave conductivity and the height integrated Pedersen conductivity in the E region. Also, the numerical study in this thesis has revealed that the eigenfrequency shifts respond to the different features of plasma density perturbations in different ways. The possibility of estimating the plasma density perturbation continuously from the IAR eigenfrequencies observed on the ground is illustrated.
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