The Contribution of N+ ions to Earth's Polar Wind

Abstract

The escape of heavy ions from the Earth atmosphere is a consequence of energization and transport mechanisms, including photoionization, electron precipitation, ion-electron-neutral chemistry, and collisions. Numerous studies considered the outflow of O + ions only, but ignored the observational record of outflowing N + . In spite of 12% mass difference, N + and O + ions have different ionization potentials, ionospheric chemistry, and scale heights. We expanded the Polar Wind Outflow Model (PWOM) to include N + and key molecular ions in the polar wind. We refer to this model expansion as the Seven Ion Polar Wind Outflow Model (7iPWOM), which involves expanded schemes for suprathermal electron production and ion-electron-neutral chemistry and collisions. Numerical experiments, designed to probe the influence of season, as well as that of solar conditions, suggest that N + is a significant ion species in the polar ionosphere and its presence largely improves the polar wind solution, as compared to observations. Plain Language Summary Nitrogen is the most abundant element in the Earth's atmosphere.Around 78% N 2 and 21% O 2 form the air we breathe and expand into high-altitude atmosphere, the thermosphere, and eventually the ionosphere. The neutral molecules in the ionosphere are ionized by solar radiation, and some of them break up into atoms, and others become charged particles. The ionospheric ions with sufficient energy can flow out into space, and the abundances of these outflowing ionospheric ions highly impact the near-Earth plasma properties. Studies focused on outflowing O + ions have been conducted for several years. However, the contribution of N + ions to the outflow solution is still largely unknown due to the instrumental limitations. This letter addresses the transport and energization of ionospheric N + ions based on theoretical predictions, and examines the role of N + ions to the collision and chemistry in the topside ionosphere. This study shows that N + ions are important components in the high-altitude polar ionosphere and their abundances can be affected by the sunlight and seasonal variations.

Stormtime Ring Current Heating of the Ionosphere and Plasmasphere

“…For example, Hoffman et al. (1974) finds that stormtime $${n}_{{\mathrm{N}}^{+}}/{n}_{{\mathrm{O}}^{+}}$$ often exceeds unity, but only at high latitudes (above 55°), where a variety of ion energization mechanisms can occur (Lin et al., 2020). The present qualitative agreement between these simulations and past data supports the hypothesis that a relatively simple thermal heating mechanism, Coulomb‐collision heating of plasmasphere electrons by ring current ions, produces both of these populations that make up the “heavy ion” (Fraser et al., 2005; Roberts et al., 1987) shell.…”

Connecting Energy Input With Ionospheric Upflow and Outflow

“…PWOM provides a first principles solution of the transport of plasma from the F‐region of the ionosphere to the magnetosphere for H + , O + , He + , and electrons. Although recent work has added N + and some molecular ions to a version of PWOM (Lin et al., 2020), these additional species are not considered in the present study. At low altitudes, below 1,000 km, the model solves the gyrotropic transport equations for each ion fluid (Gombosi & Nagy, 1989).…”