Velocity‐dispersed ion beams in the nightside auroral zone: AUREOL 3 observations

Two years of data from the AUREOL 3 satellite have been used to study in detail the energetic ion beams which precipitate at low altitudes (400–2000 km) near the poleward boundary of the nighttime auroral zone. Eighty 2–20 keV events have been identified and examined; they typically display an energy‐latitude dispersion in the auroral projection of the boundary plasma sheet, with the more energetic ions precipitating at less than 0.5° invariant latitude from the polar cap boundary. Their occurrence maximum is found in the 20–04 MLT sector and is independent of the magnetic activity measured by the AE index. In all the events studied, the ion fluxes are less than 10 7 ions cm −2 s −1 sr −1 keV −1 , and mass analysis demonstrates that they are mainly composed of energetic H + ions. All the experimental facts are consistent with an ion acceleration by the dawn‐dusk electric field in the neutral sheet with a weak magnetic field normal component B z in the distant magnetotail at an inferred distance of 50–100 R E . The observed velocity‐latitude ion segregation is the result of the E×B drift of the ion beams up to the ionosphere (geomagnetic filter effect) superimposed upon the intrinsic dispersive effect of the nonadiabatic energization mechanism, when a realistic dependence of B z on downtail distance is adopted. Finally, from the striking similarity of the experimental ion distributions we believe that the ion beams observed at low altitudes close to the polar cap boundary are the signature in the auroral ion precipitation of the earthward flowing ion beams often detected along the plasma sheet boundary layer.