Oxygen and hydrogen ion abundance in the near-Earth magnetosphere: Statistical results on the response to the geomagnetic and solar wind activity conditions

The composition of ions plays a crucial role for the fundamental plasma properties in the terrestrial magnetosphere. We investigate the oxygen-to-hydrogen ratio in the near-Earth magnetosphere from -10 RE<XGSE}< 10 RE. The results are based on seven years of ion flux measurements in the energy range ~10 keV to ~955 keV from the RAPID and CIS instruments on board the Cluster satellites. We find that (1) hydrogen ions at ~10 keV show only a slight correlation with the geomagnetic conditions and interplanetary magnetic field changes. They are best correlated with the solar wind dynamic pressure and density, which is an expected effect of the magnetospheric compression; (2) ~10 keV O+ ion intensities are more strongly affected during disturbed phase of a geomagnetic storm or substorm than >274 keV O+ ion intensities, relative to the corresponding hydrogen intensities; (3) In contrast to ~10 keV ions, the >274 keV O+ ions show the strongest acceleration during growth phase and not during the expansion phase itself. This suggests a connection between the energy input to the magnetosphere and the effective energization of energetic ions during growth phase; (4) The ratio between quiet and disturbed times for the intensities of ion ionospheric outflow is similar to those observed in the near-Earth magnetosphere at >274 keV. Therefore, the increase of the energetic ion intensity during disturbed time is more likely due to the intensification than to the more effective acceleration of the ionospheric source. In conclusion, the energization process in the near-Earth magnetosphere is mass dependent and it is more effective for the heavier ions.

💡 Research Summary

The paper presents a comprehensive statistical investigation of the oxygen‑to‑hydrogen ion ratio (O⁺/H⁺) in the near‑Earth magnetosphere, covering the region from –10 R_E to +10 R_E in the X_GSE direction. Using seven years (2001‑2007) of ion flux measurements from the RAPID and CIS instruments aboard the four Cluster spacecraft, the authors examine ion populations in two distinct energy ranges: low‑energy (~10 keV) and high‑energy (>274 keV), extending up to ~955 keV.

Data processing involved binning the measurements into 0.5 R_E intervals along the X‑axis, calculating one‑minute averaged fluxes for each bin, and correlating these with a suite of geomagnetic indices (Dst, Kp, AE) and solar‑wind parameters (dynamic pressure P_dyn, density N_sw, IMF B_z). Multivariate regression and superposed epoch analyses were employed to isolate the dominant drivers for each ion species and energy band.

Key findings are as follows:

1. Low‑energy H⁺ (~10 keV) shows only a weak dependence on geomagnetic activity. Its flux correlates most strongly with solar‑wind dynamic pressure and density, reflecting the expected response of the magnetosphere to compression. This indicates that H⁺ transport into the near‑Earth region is largely controlled by external pressure forcing rather than internal reconnection or substorm processes.

2. Low‑energy O⁺ (~10 keV) is markedly more responsive to disturbed conditions. During the main phase of geomagnetic storms and during substorm onsets, O⁺ intensities increase substantially relative to H⁺. The authors interpret this as a signature of enhanced ionospheric outflow, where O⁺ ions are lifted and subsequently accelerated by the intensified convection electric fields and reconfigured magnetic topology.

3. High‑energy O⁺ (>274 keV) behaves differently. Its strongest acceleration occurs not during the expansion phase of storms but during the growth phase, when solar‑wind energy input is gradually increasing and the cross‑polar cap potential is building up. This suggests that the energization of heavy ions is most efficient when the magnetospheric convection electric field is rising steadily, providing a prolonged acceleration window that preferentially benefits the more massive O⁺ ions. In contrast, high‑energy H⁺ shows little variation, underscoring the mass‑dependent nature of the acceleration process.

4. The ratio of quiet‑time to disturbed‑time fluxes for ionospheric outflow (derived from low‑energy measurements) matches the ratio observed for >274 keV O⁺ in the near‑Earth magnetosphere. Consequently, the increase in energetic O⁺ during disturbed periods is attributed primarily to a larger source population (i.e., intensified ionospheric outflow) rather than to a fundamentally more efficient acceleration mechanism.

Overall, the study concludes that ion energization in the near‑Earth magnetosphere is strongly mass‑dependent: heavier O⁺ ions experience more effective acceleration, especially during the growth phase of geomagnetic activity, whereas lighter H⁺ ions are mainly modulated by solar‑wind dynamic pressure. These results have important implications for magnetospheric modeling, space‑weather forecasting, and our understanding of plasma transport from the ionosphere to the magnetosphere. Future work is suggested to couple these statistical findings with global MHD‑kinetic simulations to resolve the detailed electric‑field structures and wave‑particle interactions responsible for the observed mass‑selective acceleration.