Nitrogen Ions: Game Changers of the Earth’s Ion Outflow

Knowledge of the plasma composition is key to our understanding of particle dynamics in the near-Earth region and allows us to assess how the geospace environment responds to solar wind driving. The plasma transport from the high-latitude ionosphere to the magnetosphere is known as 'ion outflow' or 'polar wind,' and its composition influences the plasma properties in the magnetosphere-ionosphere system. Since the discovery of magnetospheric oxygen ions (O+) in the 1970s, numerous studies have focused on the circulation and roles of O+ ion outflow in the magnetosphere. In addition to O+ ions, atomic nitrogen (N+) and molecular ions are also the major ion species in the Earth’s low altitude atmosphere. However, their contribution and energization mechanisms are not well understood, most likely due to the limiting capabilities of instruments flying in the space. To understand the dynamics of these heavy ion outflow, other than O+ ions, we have employed the Seven Ion Polar Wind Outflow Model (7iPWOM), a first-principles physics model, to study how they transport in the polar wind. The numerical simulations show that N+ is the second most abundant ion species up to 4,000 km altitude over the high-latitude region during summer quiet time. In spite of 12% mass difference, N+ and O+ obey different transport pathways in the polar wind, and thus, their relative abundances are varied by the locations. Moreover, the presence of N+ in the polar wind largely altered the production and energization mechanisms, which redistributed the ion composition of the polar wind. This study presents that N+ plays a key role regulating the ion outflow from hundreds of kilometers altitude to few Earth radii, and opens discussion on particle energization, loss, and transport mechanisms in the magnetosphere-ionosphere system, with the altered plasma composition.