Energization of Molecular Ions via Wave Particle Interactions in the Polar Ionosphere

The OGO-6, ISIS, and AE-C spacecraft reported the sudden enhancement of molecular ion densities at the high latitude trough (HLT). This HLT presents an abrupt change in the ratio of molecular ions to O⁺ densities, where the abundances of N₂⁺, NO⁺, and O₂⁺ increase by an order of magnitude at a time when the density of O⁺ decreases by a factor of 3. The latitude range of HLT overlaps with the polar cusp and auroral region, where energetic particle precipitation and wave activity take place. Independent observations from the Akebono, Interball-2, and Cluster satellites revealed that at few Earth radii, O⁺ ions were efficiently accelerated from 10 eV to 10 keV in just 10 minutes through resonant wave–particle interactions (WPI). Although the WPI is considered as a major pathway for heavy ion acceleration in the polar cusp and auroral region, the efficacy of WPI to the energization of molecular ion species is largely unknown.

In this study, we employ the Seven Ion Polar Wind Outflow Model (7iPWOM), and adopt a parameter study to examine the properties of N₂⁺, NO⁺, and O₂⁺ ion upflow in connection with the wave energy input. The 7iPWOM is a hybrid polar wind model which solves the transport of e⁻, H⁺, He⁺, N⁺, O⁺, N₂⁺, NO⁺, and O₂⁺, with the combination of hydrodynamics and kinetic particle-in-cell approaches. We describe the wave spectral shape by a power law profile versus frequency, as parametrized by spectral index α and the amplitude of electric field perturbation. The simulation result suggests that the molecular ions are more sensitive with the wave spectrum than N⁺ and O⁺, and presented the "valve" effect, which is a minimum wave energy to loft the molecular ions against the Earth's gravitational potential. Our findings also suggest that the wave heating immediately energizes molecular ions, and leads to the ion depletion in the low-altitude ionosphere. This in turn alters the regional ionospheric chemistry, generates molecular ions in the ionosphere, and eventually affects the polar wind dynamics globally through the polar cap convection.