See You on the Other Side: The Impact of N+/O+ Composition on Heavy Ion Transport and Magnetosphere Dynamics in Multifluid Modeling

Nitrogen ion abundances are substantial in the topside ionosphere, second only to that of oxygen ions. While N+ and O+ have similar masses, they have different ionization energies and undergo different chemical processes. Previous studies have shown that during moderate geomagnetic activity, N+ density in the inner magnetosphere rivals that of O+, and that ion composition regulates many of the magnetospheric processes. Hence, tracking the differential transport of nitrogen and oxygen ions provides insight into mass flow and energy coupling throughout the ionosphere–magnetosphere system.

Combining physics-based multifluid magnetohydrodynamic modeling and observation data of the September 7–8, 2017 storm, we assess the impact of heavy ion composition on the global magnetosphere by examining the total ion pressure, magnetotail reconnection location, and the pathways of N+ and O+ as they are transported from 2.5 RE to the nightside, as well as validation of the simulation results with plasma measurement data from Cluster, THEMIS, etc. Even a small difference in heavy ion composition at the inner boundary of the computational domain (set at 2.5 RE) leads to sizeable changes in the ion pressure on the nightside near-Earth plasma during the main phase of the geomagnetic storm, as shown in the contour in the figure. The location of the magnetotail reconnection site on the Sun–Earth line is further downtail when N+ abundances increase at the inner boundary. In addition, even under this simple assumption, the transport pathways of N+ and O+ diverge, allowing the two ion species to travel through different regions in space and opening up the possibility of experiencing different energization mechanisms.