The construction of Solar Power Satellites (SPS) would require the injection of enormous quantities of propellant to transfer cargo from LEO to the construction site at GEO. This propellant, in the form of 2 KeV argon plasma, is expected to considerably modify the space environment since the number of ions injected per SPS (=1^32) is comparable with the total plasmaspheric content above =5 km. In this thesis a model for the dynamics of the ion thruster plasma beam is presented. This model predicts beam travel distances up to a few thousand km, and the transfer of beam momentum to the ionosphere, causing localized heating. Subsequent convection of the argon is investigated using a simple model of the quite-time, and storm-time, magnetosphere. In addition, Ar+ charge exchange lifetimes are calculated and shown to be too large to significantly reduce the amount of Ar+ near the plasmapause between magnetospheric storms.