Observational approaches to the problem of magnetospheric plasma origin usually depend upon mass-discriminating measurements of ion distribution functions. A new instrument for this purpose has been developed and flown via sounding rockets above a diffuse midnight aurora during geomagnetically quiet conditions. The instrument combines wide mass and energy ranges (2.5?ME?160amu⋅keV), high mass resolution with good sensitivity, and superior rejection of undesired particles. The rocket payload approximately 0900 UT (2300 LT), traversing invariant latitudes from 65.5¿ to 67.4¿. Only ions identified as H+ and He+; were detected, and the He++ was not clearly above background, having an upper limit of 2--4% of the H+ intensity. All other ions, including the ionospheric species He+ and O+, had upper limits at a similar or smaller level. This situation contrasts sharply with a number of recently reported observations of large intensities of O+ during magnetic storms and may be characteristic of undisturbed periods. Though these results suggest a solar wind source for the ions, and admixture including an appreciable fraction of polar wind material (predominantly H+) is not precluded. The measured proton distribution was nearly isotropic over downcoming pitch angles at all energies and showed a depleted atmospheric source cone. The proton energy distribution had a best fit temperature of 4.5 keV and a number density of 0.17 cm-3, corresponding to a peak intensity just over 105 cm-2 s-1 keV-1. The energy flux carried into the atmosphere by such a distribution (0,032 erg cm-2 s-1) is consistent with groung-based H&bgr; measurement (>10R) of patches of aurora linked to the rocket by magnetic field lines. Altitudinal variations observed during the flight are consistent with the theory of charge exchange of a time steady incident proton population. These ion observations, taken together with electron observations reported in a companion paper, appear to be consistent with the model of diffuse auroras in which trapped plasma precipitates owing to strong pitch angle diffusion on auroral field lines linking the near-earth plasma sheet.