Limits on the Efficacy of Wave-Particle Interaction on the Energization and Transport of Atomic and Molecular Heavy Ionospheric Ions

Abstract

Ionospheric molecular ions, such as $N_2^{+}$, NO⁺, and $O_2^{+}$, have been observed in Earth's high-altitude ionosphere and the magnetosphere by several spacecraft missions. Their presence not only indicates that they obtain sufficient energy through effective energization mechanisms, predominantly during the geomagnetically active times, but also provides clues regarding the connection between the ionosphere and the lower thermosphere. It is, however, unknown to date which physical processes are responsible for the transport and energization of molecular ions, as well as their relative contributions to the plasma surrounding the near-Earth region. In this study, we employ the Seven Ion Polar Wind Outflow Model (7iPWOM) and examine the properties of molecular $N_2^{+}$, NO⁺, and $O_2^{+}$ upflows and outflows in response to wave activity. The 7iPWOM is a hybrid polar wind model which solves the transport of e⁻, H⁺, He⁺, N⁺, O⁺, $N_2^{+}$, NO⁺, and $O_2^{+}$, using a combination of hydrodynamics and kinetic particle-in-cell (PIC) approaches. This approach enables the inclusion of Wave-Particle Interaction (WPI) and Coulomb collisions, necessary to resolve the transport and acceleration of heavier species. The results suggest that the molecular ions are more sensitive to the wave spectrum than other ion species and exhibit a “valve” effect, meaning that a threshold wave energy is required to loft the molecular ions against the Earth's gravitational potential. Additionally, due to the limited supply of molecular ions from the ionosphere, the composition of ionospheric plasma is the primary controlling factor that regulates the abundance of molecular ion upflows and outflows.