The storm-time ring current is formed by the inward convection of the near-earth plasma sheet, so understanding the changing source of the plasma sheet is key for understanding ring current development. The ionospheric and solar wind sources can be distinguished by the charge state of the heavy ions; solar wind ions are highly iononized, while ionospheric ions are predominantly singly ionized. AMPTE/CHEM measurements are used to track the changing composition in the 6-9 Re plasma sheet as a storm develops in order to determine the fraction of the population that comes from each source. We find that prior to the storm, the solar wind source dominates. The source transitions sharply to ionospheric plasma during the storm main phase, and returns to solar wind plasma during the recovery phase.