EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind

Abstract

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° 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 (Ti < 104 °K), 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° 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.

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

“…This has been evidenced by many observational studies and models (Abe et al, 1993;Su et al, 1998a, b;Glocer et al, 2012;Maes et al, 2015). Maes et al (2015), studying outflow above smallscale polar cap arcs using Cluster (Escoubet et al, 2001) measurements, found that the upflow above the polar cap can be roughly divided into two distinct groups based on the solar zenith angle (SZA) of the footpoint of the field line in the ionosphere.…”

“…The low energization also means that it is very difficult for O + ions to escape Earth's gravitational potential via this mechanism, and so very little O + is expected in the polar wind. Nevertheless, O + ions have been observed above the polar caps and in the lobes (Nagai et al, 1984;Waite Jr. et al, 1985;Abe et al, 1993;Su et al, 1998a). Many additional mechanisms have been proposed to explain this (see, e.g., Tam et al, 2007, for an overview).…”

Ion upflow and downflow at the topside ionosphere observed by the EISCAT VHF radar

“…Recent observations with satellites such as Akebono (EXOS-D) and GEOTAIL have shown that thermal ions, including not only light ions H + and He + but also heavy ions O + and NO +¯o w out of the polar ionosphere and they are found in the lower magnetosphere (Watanabe et al, 1992;Abe et al, 1993a, b;Yau et al, 1991Yau et al, , 1993Yau et al, , 1995 and in the lobe magnetosphere (Mukai et al, 1994;Hirahara et al, 1996;Seki et al, 1996Seki et al, , 1998. A very comprehensive review of the terrestrial plasma source obtained from the DE mission was given by Chappell (1988).…”

Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

“…It is known that oxygen ions of low charge state are originated from the terrestrial ionosphere (cf. Abe et al, 1993;André and Yau, 1997;Nilsson et al, 2006) and that they follow various fates; for instance, some of them escape into the distant tail, and…”