The polar cap environment of outflowing O+

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Ion composition measurements by Dynamics Explorer 1 often show upward O+ beams at polar latitudes, with streaming energies of 1–20 eV or more. Here we utilize measurements of core (0–50 eV) and “energetic” (∼0–1 keV) ion composition, plasma waves, and auroral images from DE 1 and plasma ions and electrons from DE 2 to examine some of their properties in the context of the polar cap environment. It is found that two distinct populations of O+ beams are observed: “high‐speed” (10–30 eV or higher streaming energies) and “low‐speed” (generally <10‐eV streaming energies). The “high‐speed” polar beams show an “auroral” connection; i.e., they are observed on or near field lines threading auroral arcs seen in DE 1 images. The “low‐speed” streams are on or near field lines threading the dark polar cap and may be convected from the cleft ion fountain. The low‐speed streams are generally much more stable in energy and flux, while the high‐speed streams tend to be bursty. In general, the streams are convecting antisunward, with velocities of 5–14 km/s in the orbital plane. We sought to obtain plasma density estimates from plasma wave measurements, through analysis of features of auroral hiss as well as upper hybrid emissions. Densities in the range 1–5 el/cm³ were indicated for one segment of a DE 1 polar cap pass; however, the measurements generally indicate little auroral hiss or upper hybrid emissions in the polar cap for the other cases considered here. Estimates of electrostatic potential drops above the DE 2 satellite have been made using energy‐angle spectrograms of photoelectron data, under the assumption that the field lines of observation are “effectively” open. Potential drops often are in the 20‐ to 40‐V range. At other times the potential falls below the ∼5‐V instrument threshold, or there are insufficient photoelectron fluxes for estimation. These limited data suggest that the largest potential drops are just poleward of the cleft or near its poleward edge and there is a decline of the drop in the antisunward direction. No obvious correlation between the potential estimates and “nearby” O+ streaming energies is seen.