Since December 2000, the Cluster satellites have beenconducting detailed measurements of the magnetospheric boundaries andhave confirmed the unambiguous presence of ions of terrestrial origin (e.g.O in regions adjacent to the dayside, mid-latitude magnetopause. Inthe present paper, we focus on the statistical properties of the O ioncomponent at energies ranging from 30eV up to 40keV, using three years ofion data at solar maximum from the Cluster Ion Spectrometry (CIS) experimentaboard two Cluster spacecraft. The O density decreases on average by afactor of 6, from 0.041 to 7x10cm when crossing themagnetopause from the magnetosphere to the magnetosheath, but depends onseveral parameters, such as the geomagnetic activity or the modifieddisturbed storm time index (*), and on their location. The Odensity is significantly higher in the dusk-side than in the dawn side region,which is consistent with the view that they originate mainly from the plasmasheet. A remarkable finding is that inward of the magnetopause, O isthe dominant contributor to the mass density 30% of the time on thedusk-side in comparison to 3% in the dawnside and 4% near noon. On an eventbasis in the dusk flank of the magnetopause, we point out that O ions,when dominating the mass composition, lower the threshold for generating theKelvin-Helmholtz instability, which may allow plasma exchange between themagnetosheath and the plasma sheet. We also discuss the effect of asubstantial O ion component when present in a reconnection region.
展开▼
