Survey of oxygen ions and mass density in the dayside near-magnetopause region

Since december 2000, The Cluster spacecraft have been conducting detailed measurements of the magnetospheric boundaries and confirmed the unambiguous presence of ions of terrestrial origin (e.g. O+) in regions adjacent to the dayside, mid-latitude magnetopause. The present paper focusses on the statistical properties of the O+ ion component at energies ranging from 30 eV up to 40 keV using three years of ion data at solar maximum from the Cluster Ion Spectrometry (CIS) experiment aboard two Cluster spacecraft. The O+ density decreases on average by a factor of 6 when crossing the magnetopause from the magnetosphere to the magnetosheath, but depends on several parameters such as the geomagnetic activity or the disturbed storm time index, and on their location. The O+ density is significantly higher in the duskside than in the dawnside region, which is consistent with the view that they originate mainly from the plasma sheet. A remarkable finding is that inward of the magnetopause, O+ is the dominant contributor to the mass density 30% of the time on the duskside as against 3% in the dawnside and 4% near noon. The O+ ion density in the adjacent magnetosheath is found to be slightly higher when the interplanetary magnetic field (IMF) is northward rather than southward, indicating that energetic ions are always escaping whether or not conditions support dayside reconnection. This result can also be explained in terms of convection drift paths; the presence of sunward drift velocities under northward IMF helps ions from dayside sources to leak out across the dayside magnetopause while in contrast, antisunward drift velocities under southward IMF would prevent such ions from escaping. We also discuss the role of a substantial O+ ion component in affecting physical processes such as the Kelvin-Helmholtz instability or magnetic reconnection.