Assessment of Efficacy of WPI in the Vertical Transport of Ionospheric Heavy Ions

Singly charged heavy ions observed in the magnetosphere, such as atomic N⁺, O⁺, and molecular N₂⁺, NO⁺, and O₂⁺ ions, are sourced from the Earth’s ionosphere. Their escape is facilitated by various energization mechanisms, such as photoionization, Suprathermal Electron (SE) impact, ion–electron–neutral chemistry and collisions, and wave–particle interactions (WPI). The presence of the heavy ions in the Earth’s magnetosphere–ionosphere system impacts the dynamics and morphology of the near-Earth plasma, and provides clues regarding the connection between the ionosphere with the lower thermosphere. The existing observational records suggest that N⁺ are constant companions of outflowing O⁺ ions, and during geomagnetically active times, the increase in molecular ions were often accompanied by a high ratio of N⁺/O⁺ in the Earth’s magnetosphere–ionosphere system. It is however unknown to date which physical processes are responsible for the transport and energization of N⁺ and molecular ions, as well as their relative contributions to the plasma surrounding the Earth.

We employ the Seven Ion Polar Wind Outflow Model (7iPWOM), which solves for the hydrodynamics transport equations for e⁻, H⁺, He⁺, N⁺, O⁺, N₂⁺, NO⁺, and O₂⁺ below 1,000 km altitude, and includes advanced schemes for SE impact and competing chemical reactions. Above 1,000 km, it applies a transition to a kinetic particle-in-cell (PIC) solution, which enables the inclusion of WPI and Coulomb collisions, necessary to resolve the transport and acceleration of heavier species. We examine the role of solar illumination, solar wind impact, and neutral atmosphere on the dynamics of polar wind plasma. Numerical simulations suggest that the presence of N⁺ redistributes the ion composition of polar wind by altering the chemistry and SE production at low-altitude ionosphere and increases the overall escape rates of the polar wind. Moreover, the contribution of N⁺ to the dynamics of polar wind plasma varies with the seasons and solar cycles due to the different compositions of the neutral atmosphere and the variation in the solar EUV flux. Furthermore, WPI are known to preferentially heat the heavy ions, as their gyro-frequency is more likely to be resonant with the low frequency wave. We show that molecular ions can acquire sufficient energy via WPI to be lofted, and further quantify the efficacy of WPI to outflowing heavy ions in response to various conditions. This indicates that tracking the heavy ions, particularly for the molecular ions, could provide a reference to understand the wave heating mechanisms in the magnetosphere–ionosphere system.