N+ and Molecular Ion Composition in Topside Auroral Upflows and Downflows

The high latitude ionosphere is an important site of coupling between the magnetosphere, ionosphere, and thermosphere (MIT): ion upflows and outflows from the topside auroral ionosphere are an important source of plasma in the magnetosphere. To date, studies of ion upflows and downflows have been focused primarily on O⁺ ions, which are the dominant ion species in the F-region and topside ionosphere. In comparison, nitrogen and molecular ions are generally assumed to be minor species, and they have received relatively little attention in such studies, in part due to the difficulty in resolving N⁺ from O⁺ in ion composition measurements.

We present composition measurements of ion upflows and downflows in the topside auroral ionosphere (between 325 and 1500 km) from the e-POP (Swarm-E) Imaging and Rapid Scanning Mass Spectrometer (IRM), which combines the techniques of hemispherical electrostatic analysis and ion time of flight (TOF) to resolve both the energy-per-charge (E/q) and mass-per-charge (M/q) of observed ions at ~0.5 - 90 eV/q. We present a statistical study of the composition distributions of both ion upflows and downflows, in which we exploit the capability of IRM to unambiguously identify the presence of N⁺ and molecular ions (N₂⁺, NO⁺ and O₂⁺), and discuss the occurrence and source mechanisms of enhanced N⁺ and molecular ion densities at both geomagnetically active and quiet times.