Observations of polar wind and thermal ion outflow by akebono/SMS

Abstract

We present a statistical analysis of thermal H+ and O+ ion flux measurements in the high-altitude (6000~9000 km) polar ionosphere from the Suprathermal ion Mass Spectrometer (SMS) on Akebono. It is shown that the normalized H+ polar wind flux (to 2000 km altitude) varies from 107 to 108 cm-4s"1 at 2000 km altitudes. Surprisingly, the O+ ion flux is found to be comparable to the H+ ion flux and much higher than classical theory prediction. The magnetic local time (MLT) distribution of the upward ion flux and its geomagnetic activity (Kp) dependence are also presented. At both magnetically quiet and active times, the integrated H+ ion flux is largest in the noon sector (09-15 MLT) and smallest in the midnight sector (21-03 MLT); the flux ratio was found to be approximately one order of magnitude. The total flux of H+ ion outflow integrated over the polar ionosphere (ILAT > 75°) and over all local times was found to correlate inversely with the Kp index. The integrated H+ flux (ILAT > 75°) in quiet times was 0.9-1.5 x 1025ionss~' while the flux in active times was a factor of 2-3 smaller (0.4-0.6 x 1025 ionss"1). It also exhibited a slight positive correlation with the IMF (interplanetary magnetic field) Bz component.