HF Doppler observations of ULF waves: System development and high latitude results.

This thesis describes a study of ultra low frequency (ULF) waves in the high latitude ionosphere. These waves have been observed by means of a high frequency (HF) Doppler sounder known as the Doppler Pulsation Experiment (DOPE). The ULF waves perturb the phase path which produces a small Doppler frequency shift in the received radio signal. These Doppler signatures can be related to the ULF wave characteristics. The receiver is computer controlled and dedicated software enables the system to run unattended for several weeks. The archived data are returned to Leicester for analysis. The sounder is located in northern Norway near the EISCAT radar installation. A statistical study of an earlier data set collected at the same location has established the diurnal, seasonal, solar cycle and geomagnetic activity variations of the ULF wave signatures. The effectiveness of the technique for observing ULF waves has also been investigated and this has allowed the optimisation of DOPE for studies of ULF waves. The first results from DOPE indicate that two distinct types of waves are present; those which have a correlated ground magnetic signature and those which do not. The "correlated" events are associated with field line resonances in the Earth's magnetosphere. The "uncorrelated" signatures are divided into events which occur in the morning and afternoon sectors. The morning events are reminiscent of giant pulsations which occur under quiet geomagnetic conditions. Afternoon waves coincide with depressions of the Dst index and are consistent with observations of storm time ULF waves. The "uncorrelated" waves are not observed by ground based magnetometers because of their high azimuthal wave numbers. The Doppler technique has proved to be a powerful tool for ULF wave studies and a number of ULF wave features have been identified for the first time.