EXOS-D (AKEBONO) SUPRATHERMAL MASS-SPECTROMETER OBSERVATIONS OF THE POLAR WIND

We report observations of the H+, He+, and O+ polar wind ions in the polar cap(>80-degrees invariant latitude. ILAT) above the collision-dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low-energy (a few eV) upward ion flows in the high-altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field-aligned and cold (T(i) < 10(4) K-degrees), and the parallel (field-aligned) velocities of the different ion species vary inversely with the respective ion masses. A statistical study of the altitude, invariant latitude, and magnetic local time distributions of the parallel velocities of the respective ion species is described, and preliminary estimates of ion temperatures and densities, uncorrected for perpendicular drifts and spacecraft potential effects, are also presented. For all three ion species, the parallel ion velocity increased with altitude. In the high-latitude polar cap (>80-degrees ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s. respectively. Both the ion velocity and temperature distributions exhibited a day-to-night asymmetry, with higher average values on the dayside than on the nightside.