Akebono Observations of the Polar Wind and Suprathermal Auroral Ions: An Overview.

Abstract

We present an overview of direct ion and electron observations of the polar wind and suprathermal auroral ions from the plasma instruments on Akebono. In the polar wind, the observed H+ ion reaches a velocity of 1 km/s at 2000 km altitude, as does He+ ion near 3000 km and O+ ion near 6000 km. At high altitudes, the O+ ion constitutes a significant component of the polar wind flow contrary to most polar wind model predictions. At 2000-4000 km, the H+ ion velocity in the polar wind is strongly correlated with the ambient electron temperature. The electron temperature (averaged thermal energy) in the upward magnetic field direction is a factor of 1.5-2 higher than that in the downward and field-perpendicular directions. This results in an upward heat flux which is comparable in magnitude to the heat flux carried by energetic (>10-eV) photoelectrons. The observed electron temperature anisotropy and correlation with ion velocity are consistent with the polar wind being driven by a large ambipolar electric field resulting from the presence of escaping atmospheric photoelectrons (required to maintain quasi-neutrality along the field line). At auroral latitudes, significant fluxes of “minor ions” are frequently observed, particularly atomic N+ and O++ ions and molecular NO+, N2+, and O2+ ions. The occurrence of molecular ions at high altitude (>3000 km) during extended periods of auroral activity points to their fast energization at the F-region and topside ionosphere. In the 3000-6000 km altitude region, transverse ion energization occurs frequently both on the dayside and the nightside. Different ion species are energized in the perpendicular direction to tens and at times hundreds of eV over a narrow spatial extent, and appear as “ion conics” at higher altitudes as they spiral up the field lines. In the 3000-9000 km altitude region, the average energy of the observed ion conics in the dayside auroral zone increases with altitude.