Data obtained during an active rocket-borne ion beam injection experiment (ARCS 3), which was carried out in the polar cap ionosphere over Sondre Stromfjord, Greenland, are presented and discussed. The experiment consisted of a main instrumented sounding rocket payload and a separated subpayload from which two Ar+ ion beam generators were operated. These generators produced neutralized ion beams which were aligned generally perpendicular and antiparallel to the ambient geomagnetic field lines, as the two spinning payloads moved apart along (0--1 km) and across (0--0.5 km) field lines. Ion flux data obtained aboard the main payload show certain characteristics which are directly attributable to known beam-injection geometry parameters, and others which are not so easily understood. Specifically, when the beam is injected transverse to the geomagnetic field, strong ion fluxes near 90¿ magnetic pitch angle are observed, as expected. The particle energies in these cases were different from those expected, however, with separate ion flux components appearing near 100 and 15 eV/q.

The expected beam energy was near 200 eV/q, so that subpayload charging to levels on the order of 100 V is inferred to have taken place during transverse beam injections. During antiparallel injections a component of ~200 eV/q ions is observed to be moving up the field line, in addition to two other components: one transverse component at ~200 eV/q and a second transverse component near 15 eV/q. In this case, no spacecraft charging is inferred. The mechanism giving rise to the 15-eV/q transverse ion components, observed during both transverse and antiparallel injections, is an open question. The existence of ion fluxes at magnetic pitch angles other than those anticipated during both transverse and antiparallel beam injections indicate ion emissions from the subpayload which are much less beamlike than expected based on preflight laboratory measurements and that beam-plasma interactions tend to isotropize the beam very near the emitting payload. In addition, photometric, electric field, and ion flux data are presented which support the picture of a spatially asymmetric and highly localized region of strong beam-plasma interactions surrounding the beam-emitting subpayload. ¿ American Geophysical Union 1988