We present a comprehensive study of plasma properties at geosynchronous orbit for electron and ion energies between ~1 eV and ~45 keV, between 1990 and 2001. The variations of temperature and density are analyzed as functions of local time, magnetospheric convection strength, and the strength of the ring current. Various parameters derived from temperature and density are calculated to elucidate the temporal and spatial location of delivery of plasma sheet material into the inner magnetosphere. We find that the electron and proton densities are greatest in the dawn region for the periods of highest convection and ring current strength. We perform a superposed epoch analysis of 283 geomagnetic storms which occurred between 1991 and 2001 and examine the temporal variation of the plasma at geosynchronous orbit as a function of storm phase. This analysis demonstrates the local time variability of delivery from the plasma sheet into the inner magnetosphere and the concurrent changes in temperature and pressure. We demonstrate that the density of electrons in the plasma sheet is strongly dependent upon the magnetospheric convection strength and, for the first time, upon solar activity. Electron density at geosynchronous orbit is strongly correlated with solar activity. The average plasma sheet electron density at solar maximum can be a factor of two or more higher than that at solar minimum. We also outline a method to estimate the composition of the plasma sheet from MPA measurements and calculate the O+ and H+ density variations with solar cycle as a function of Kp and local time. We show that the O+ and H+ plasma sheet densities increase with increasing solar activity, as does the O+/H+ density ratio. During times of high solar activity and strong convection, the O+ and H+ densities may be comparable.