A burst of energetic O⁺ ions during an upstream particle event

During an extended energetic particle event upstream of the earth's bow shock on September 5, 1984, a brief burst of energetic O + ions has been observed using the time‐of‐flight spectrometer SULEICA on the AMPTE/IRM spacecraft. The event occurred during very active geomagnetic conditions. The onset of the O + event coincides with a brief electron burst. O + is predominantly streaming into the sunward direction, while the H + and He 2+ ions exhibit a net flow toward the bow shock with a second order anisotropy perpendicular to the interplanetary magnetic field (which was almost radial throughout the whole event). The energy spectrum of O + is significantly harder than those of H + , He 2+ and CNO (Q>2). The observations of two particle populations, different in temporal evolution, anisotropies and energy spectra, are discussed in terms of bow shock accelerated ions over the entire time period accompanied by a brief injection of magnetospheric particles.