Survey of energetic O + ions near the dayside mid-latitude magnetopause with Cluster

Since December 2000, the Cluster satellites have been conducting detailed measurements of the magneto- spheric boundaries and have confirmed the unambiguous presence of ions of terrestrial origin (e.g. O + ) in regions adjacent to the dayside, mid-latitude magnetopause. In the present paper, we focus 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 Clus- ter spacecraft. The O + density decreases on average by a fac- tor of 6, from 0.041 to 7◊10 3 cm 3 when crossing the mag- netopause from the magnetosphere to the magnetosheath, but depends on several parameters, such as the geomagnetic ac- tivity or the modified disturbed storm time index ( Dst ), and on their location. The O + density is significantly higher in the dusk-side than in the dawn side region, which is consis- tent with the view that they originate mainly from the plasma sheet. A remarkable finding is that inward of the magne- topause, O + is the dominant contributor to the mass density 30% of the time on the dusk-side in comparison to 3% in the dawnside and 4% near noon. On an event basis in the dusk flank of the magnetopause, we point out that O + ions,