This thesis is concerned with the open magnetic flux content of the magnetosphere\ud during the substorm cycle.\ud We develop an automated technique of identifying the global location of the\ud open/closed magnetic field line boundary (OCB) from auroral images of the\ud ionospheric northern polar cap made by the IMAGE FUV detector, and hence of\ud estimating the open magnetic flux content of any one hemisphere (Fpc). Systematic\ud offsets are found between the location of the OCB as estimated from the auroral\ud images and from in-situ measurements of precipitating particles consistent with\ud previous studies. These offsets are used to provide a more accurate global\ud estimation of the location of the OCB.\ud This method is used to estimate the average Fpc content of the magnetosphere from\ud 12,731 auroral images from December and January of 2000-2002, 173 of which\ud occur at the time of substorm onset. From these distributions we are able, for the\ud first time, to determine the probability of substorm onset as a function of Fpc. The\ud probability of substorm onset is found to be negligible below ~ 0.3 GWb, increases\ud linearly until ~0.9 GWb, and is undefined above this. These results are discussed in\ud terms of various models for substorm onset and place important constraints on any\ud future models.\ud We also find that substorms showing a clear particle injection signature, as seen at\ud geosynchronous orbit, occur, on average, at higher values of open magnetic flux\ud than those showing varied or no injection activity. In the final part of this thesis we\ud investigate these three distinct categories of substorms further by carrying out a\ud superposed epoch analysis of Fpc, maximum nightside auroral brightness, solar wind\ud and IMF conditions, geomagnetic activity and auroral oval boundary motions. We\ud find that the level and form of solar wind driving appears to characterise the\ud injection signature seen
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