Ion drag is known to play an important role in driving neutral thermosphere circulation at auroral latitudes, especially during the main phase of geomagnetic storms. During the recovery phase, the neutrals are known to drive the ions and generate ionospheric electric fields and currents via the disturbance dynamo mechanism. At midlatitudes, the precise interplay between ions and neutrals is less understood largely because of the paucity of measurements that have been available. In this work, we investigate ion‐neutral coupling at middle latitudes using colocated ion drift velocity measurements obtained from Super Dual Auroral Radar Network radars and neutral wind velocity and temperature measurements obtained from the North American Thermosphere Ionosphere Observing Network (NATION) Fabry‐Perot interferometers. We examine one recent storm period on 2–3 October 2013 during both the main phase and late recovery phase. By using ion‐neutral momentum exchange theory and a time‐lagged correlation analysis, we analyze the coupling time scales and dominant driving mechanisms. We observe that during the main phase the neutrals respond to the ion convection on a time scale of ∼84 min which is significantly faster than what would be expected from local ion drag momentum forcing alone. This suggests that other storm time influences are important for driving the neutrals during the main phase, such as Joule heating. During the late recovery phase, the neutrals are observed to drive the ion convection without any significant time delay, consistent with the so‐called “neutral fly wheel effect” or disturbance dynamo persisting well into the late recovery phase.

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Measurements of the limiting escape rate of atomic hydrogen (H) atoms at Earth and the relative significance of thermal evaporation and nonthermal escape mechanisms, such as charge exchange and polar wind, have long been lacking. Our recent development of sophisticated radiative transport analysis techniques now enables the reliable interpretation of remotely sensed measurements of optically thick H emission, such as those acquired along the Earth's limb by the Global Ultraviolet Imager (GUVI) onboard the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft, in terms of physical parameters such as exobase density and, crucially, vertical diffusive flux. In this work, we present results from a systematic investigation of H Ly a emission measured by TIMED/GUVI along the Earth's dayside limb from 2002-2007, which we use to derive the vertical H flux and associated density distribution from 250 km out to 1 Earth radius. Our analysis reveals that the vertical flux of thermospheric H is nearly constant over a large range of solar activity and typically exceeds the calculated thermal evaporative flux, suggesting that terrestrial H escape is indeed limited by its vertical diffusion. The excess supply of H atoms to the exobase associated with large observed vertical fluxes requires that nonthermal escape mechanisms be operative for steady-state continuity balance. We find that such nonthermal processes are a particularly significant component of total H escape during low solar activity, when thermal evaporation is weakest.

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, Q O−O + , 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 Q O−O + , 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.

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