Extensive ground-based arrays of magnetometers and numerous satellite platforms in the outer magnetosphere have established that two separate substorm expansion onsets occurred on March 22, 1979. The first of these occurred at 1055 UT and is demonstrated to be localized in the 0200--0300 LT sector. Concurrent plasma sheet ion composition measurements are used to show that the growth and expansion phase of this substorm occurred while the outer magnetosphere was composed dominantly of solar wind (H+ and He++) plasmas. The 1055 UT substorm greatly perturbed and altered the ion composition of the plasma in the outer magnetosphere such that the second substorm expansion onset (1436 UT) occurred while the outer magnetospheric plasmas were dominantly of ionospheric (O+) origin. The 1436 UT substorm is shown to have a component of the westward electrojet localized further westward in local time relative to the first substorm. These results are a consistent, well-documented example of the possible important role of heavy ions in the localization and initiation of plasma sheet instabilities during substorms.