Survey of energetic O<Sup>+</sup> ions near the dayside mid-latitude magnetopause with Cluster

Abstract

Abstract. Since December 2000, the Cluster satellites have been conducting detailed measurements of the magnetospheric 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 Cluster spacecraft. The O + density decreases on average by a factor of 6, from 0.041 to 7×10 −3 cm −3 when crossing the magnetopause from the magnetosphere to the magnetosheath, but depends on several parameters, such as the geomagnetic activity or the modified disturbed storm time index (D st * ), and on their location. The O + density 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 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 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, when dominating the mass composition, lower the threshold for generating the Kelvin-Helmholtz instability, which may allow plasma exchange between the magnetosheath and the plasma sheet. We also discuss the effect of a substantial O + ion component when present in a reconnection region.

Atmospheric loss from the dayside open polar region and its dependence on geomagnetic activity: implications for atmospheric escape on evolutionary timescales

“…Other studies that have shown a clear correlation between O + and geomagnetic activity are Peterson et al (2001), Cully et al (2003), and Kistler and Mouikis (2016). The O + density close to the mid-latitude magnetopause was shown by Bouhram et al (2005) to also increase exponentially with Kp. A consequence of increased ion outflow is an enhancement of the plasma feed into the plasma sheet during geomagnetic storms (Nose et al, 2005;Kistler et al, 2010;Haaland et al, 2012).…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…Modulation of the O + density and energy by geomagnetic activity monitored by the Sym-H index was shown by Ohtani et al (2011). The O + density just inward of the magnetopause in the magnetospheric boundary layer also displays a significant exponential increase with geomagnetic activity indicated by the Dst index (Bouhram et al 2005).…”

“…This variability explains why O + can be the most abundant ion during specific periods in the midnight sector (Lennartsson and Shelley 1986;, in the near-Earth regions (e.g. Maggiolo and Kistler 2014) or in the dayside magnetosphere (Bouhram et al 2005) while if we consider the average plasma sheet composition H + is the dominant species (in number density) for all geomagnetic and solar activity level except at low geocentric distances near geostationary orbit .…”

“…+ Energy The typical O + energy in the plasma sheet has been estimated to be around 3-4 keV both in the dayside magnetosphere (Bouhram et al 2005) and in the tail plasma sheet (Lennartsson and Shelley 1986;Lennartsson 1989). Ohtani et al (2011) provide much higher estimates of the O + energy (up to 10-20 keV) but from the extrapolation of high energy measurements, which as conceded by the authors may be associated with some errors.…”

“…This diversity may be result from the different energy ranges included in the different studies (for instance Lennartsson and Shelley 1986 considered energies below 1 keV) or from the data selection method (see discussion in . At the magnetopause, Bouhram et al (2005) showed a strong dawn dusk asymmetry in the average O + density below 40 keV which is higher in the dusk side (on average 0.053 cm −3 which represents on average 30 % of the mass density) than in the dawn side (0.014 cm −3 , corresponding to only 3 % of the mass density). For energies above 274 keV higher fluxes are observed on the duskside and interpreted it as being the result of significant loss of energetic ions in the dayside, as illustrated in Fig.…”