Studies of nightside spectral with behaviour from coherent high frequency radars

The work presented in this thesis is aimed to improving our understanding of the HF Doppler spectral width parameter and how it can be used to identify ionospheric and magnetospheric boundaries, specifically the use of the frequently observed gradient between high (>200 m s-1) and low (<200 m s-1) spectral width. Locating the boundaries between regions is important to the study of how the magnetosphere responds to the solar wind and interplanetary magnetic field conditions. Three case studies and a statistical investigation are presented herein which investigate the nature of these regions. Data from the Co-operative U.K. Twin Located Auroral Sounding System (CUTLASS) HF radars and the European Incoherent SCATter (EISCAT) radars from the primary observations. The cases presented show that high spectral width can be observed both on open and closed magnetic field lines, and that there is a relationship between elevated electron temperature and high spectral width (although the reverse is not true) which appears to be restricted to the post 0300 MLT nightside region. The data also show that high spectral widths can be associated with both single- and multiple-peak HF spectra. The statistical study compares three years of data from the CUTLASS radars and the Syowa East radar (part of the Super Dual Auroral Radar Network, SuperDARN), magnetically conjugate to Iceland East. It is found that the mean spectral width is dependent on latitude, magnetic local time (MLT) and season. The data from the two hemispheres show similar dependence on these factors, although the Syowa East spectral widths are larger in general (instrumental variations are discounted). These results suggest that the physical mechanism(s) creating the high spectral widths must work both on open and closed field lines, be dependent upon latitude and MLT, and be less prevalent (or attenuated) in summer months.