Storm time empirical model of O + and O 6+ distributions in the magnetosphere

Abstract Recent studies have utilized different charge states of oxygen ions as a tracer for the origins of plasma populations in the magnetosphere of Earth, using O + as an indicator of ionospheric‐originating plasma and O 6+ as an indicator of solar wind‐originating plasma. These studies have correlated enhancements in O 6+ to various solar wind and geomagnetic conditions to characterize the dominant solar wind injection mechanisms into the magnetosphere but did not include analysis of the temporal evolution of these ions. A sixth‐order Fourier expansion model based empirically on a superposed epoch analysis of geomagnetic storms observed by Polar is presented in this study to provide insight into the evolution of both ionospheric‐originating and solar wind‐originating plasma throughout geomagnetic storms. At high energies (~200 keV) the flux of O + and O 6+ are seen to become comparable in the outer magnetosphere. Moreover, while the density of O + is far higher than O 6+ , the two charge states have comparable pressures in the outer magnetosphere. The temperature of O 6+ is generally higher than that of O + , because the O 6+ is injected from preheated magnetosheath populations before undergoing further heating once in the magnetosphere. A comparison between the model results with O + observations from the Magnetospheric Multiscale mission and the Van Allen Probes provides a validation of the model. In general, this empirical model agrees qualitatively well with the trends seen in both data sets. Quantitatively, the modeled density, pressure, and temperature almost always agree within a factor of at most 10, 5, and 2, respectively.