Ionospheric O⁺ Momentum Balance Through Charge Exchange With Thermospheric O Atoms

One of the most notorious uncertainties in ionosphere/thermosphere physics concerns the cross section for resonant charge exchange between neutral atomic and ionized oxygen, O and O⁺, the principle constituents between ∼200–500 km in the terrestrial atmosphere. O − O⁺ charge exchange plays a vital role in both momentum and energy exchange between the thermosphere and ionosphere, such that the value of the cross section, (Formula presented.), strongly influences calculations of plasma drift speeds, diffusion coefficients, and electron density distributions. We present an analysis of the nighttime O⁺ momentum budget in the F region ionosphere, using an unprecedented 27-year baseline of observations from Arecibo Observatory, as a means to assess the agreement between the data, recent theoretical calculations of (Formula presented.), and NRLMSISE-00 model predictions of O density, [O]. We find evidence for local time, seasonal, and solar cycle variation between the derived and modeled O − O⁺ collision frequency, with the best agreement observed near midnight at solar maximum. The data overall support our conclusion that recent theory regarding the magnitude of Q_O−O⁺ is likely accurate. Although biases in the momentum balance technique may account in part or in full for the local time and seasonal dependencies in observed O⁺ momentum imbalances, the most likely source of the observed solar cycle variation is MSIS model underestimation of [O]. These findings serve to establish the O⁺ momentum and energy balance techniques as a valuable means of remotely sensing thermospheric [O] in support of future model validation and development.