| Summary: | The work presented in this thesis is intended to improve our understanding of the magnetospheric substorm process and in particular the substorm onset mechanism. Studying the timing of known substorm expansion phase onset signatures from a variety of instruments collecting data in key regions of the magnetosphere, ionosphere and on the ground allowed comparison of the events chosen to substorm onset models. Two substorm case studies are presented for intervals during which complementary data sets were available. The principal data sets came from the Fluxgate magnetometer (FGM), Plasma Electron and Current Experiment (PEACE), Cluster Ion Experiment (CIS) and Research with Adaptive Particle Imaging Detectors (RAPID) instruments on the Cluster spacecraft. It was only possible to determine the onset mechanism for one of the three substorms studied (the near-Earth neutral line model). Rapid thinning of the current sheet was detected prior to the third substorm studied. A tailward-moving travelling compression region was then detected in the magnetotail at substorm onset, and the near-Earth neutral line was located at 15 RE downtail. At onset the Region 1 current system was skewed to the pre-midnight sector, later returning to a more central location about the midnight sector. A new feature has been identified in the magnetotail and termed a magnetic cavity. This was not a singular feature; ten magnetic cavities were identified and four studied in detail. They are features located in the plasma sheet boundary layer and plasma sheet which have close to zero magnetic field and plasma characteristics similar to that of the central plasma sheet. There is evidence that magnetic cavities may be associated with the recovery phase of magnetospheric substorms.
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