The plasma characteristics of O+, H+ and He+ beam events in the polar cap region observed near solar maximum by the Dynamics Explorer 1 (DE 1) satellite are analyzed in this paper. The data analyzed were taken from both the energetic ion composition spectrometer (EICS), which operates in the 10 eV--17 keV energy range, and the retarding ion mass spectrometer (RIMS), which operates in the low energy spectrum of less than 50 eV. Electron densities measured by the plasma wave instrument (PWI) provided limits on estimates of ion densities. Because the upflowing ion energies are sometimes comparable to the spacecraft potentials and the fluxes involved are close to the instrumental thresholds, it has been difficult to measure the densities and temperatures of these low energy ions. A subset of upflowing polar cap ion streams was identified from which it was possible to make estimates of the plasma characteristics using the three DE 1 instruments described above. In this paper, the data for two polar ion streams are presented in detail. The plasma was found to consist of multiple streaming and quasi-isotropic components.

In both cases, estimates of temperatures and densities were made taking into account possible spacecraft potentials. The ion densities of several other events were also estimated. It was found that (1) the plasma often had a large content of upflowing O+ ions, (2) there was a significant amount of O+ in the plasma even during quiet auroral conditions when the AE index was only 37 nT, (3) in one event presented, the upflowing O+ population had both a cold and a warm field-aligned distribution, (4) in another event presented, the O+ and H+ temperatures (for an assumed spacecraft potential of 5 eV) were estimated to be 5 eV and 7.6 eV, respectively, suggesting that the ionospheric ions were heated, (5) the cold upflowing ion stream component observed in some of the polar ion streaming events exhibited a filamentary nature (with a time scale ranging from tens of seconds to minutes, which corresponds to spatial scales of tens to hundreds of kilometers in the ionosphere) and (6) there also often was a significant amount of He+ found in the plasma. The observations provide important plasma characteristics of the polar cap region, which will be useful in future theoretical and simulation work. ¿ American Geophysical Union 1990