Magnetospheric Oxygen in the Magnetosheath as Observed by CIS Cluster

The Cluster Ion Spectrometry experiment (CIS) on board CLUSTER mission permits high time resolutions measurements of the three dimensional distribution functions for the four major ion species H+, He++, He+, O+. Here we present Cluster observations made on the dusk flank of the magnetopause in which, for a long time interval, magnetospheric oxygen was observed outside the magnetosphere. The oxygen distribution functions are characterized by noteworthy variability along the period of observations. We investigate on the most probable cause for the oxygen escaping process and discuss possible explanations for the peculiarities observed in the distribution functions.