This proposal focuses on how the expansion phase of a substorm [as first described by Akasofu, 1964] is triggered. The expansion phase is the period of time in which there exist extremely large electric fields and high fluxes of energetic particles in the ionosphere. The mechanism which triggers the start of the expansion phase is still a mystery, after many years of research, although many interesting and well thought out theories have been put forth. The investigation proposed here will focus on determining the influence of (1) the interplanetary magnetic field, and (2) the ionospheric conductivity on the expansion phase onset. This is an important proposal because it will address a number of fundamental issues in the field of substorm expansion phase onsets.
