Quantification of upper thermosphere composition and its dynamic charge exchange coupling to the mid-latitude topside ionosphere

Abstract:

A key mechanism of dynamic coupling between the upper thermosphere and topside ionosphere is the resonant charge exchange between its major constituents: ionized and neutral atomic hydrogen and oxygen. As the dominant chemical source and sink for protons in particular, deviations from charge exchange equilibrium are associated with neutral composition changes as well as vertical plasma flows, both diurnally and in response to sporadic, storm-driven disturbances. The precise light ion partitioning by the incoherent scatter radar (ISR) at Arecibo Observatory in Puerto Rico enables empirical quantification of this charge exchange coupling in terms of neutral O and H density, such that specification of one neutral species allows estimation of the other. Unfortunately, the historical lack of reliable quantification of either species' density has limited the routine application of this technique until now. Recent success in deriving precise estimates of H density throughout the thermosphere, based on measurements of its Lyman alpha airglow emission acquired by the Global Ultraviolet Imager (GUVI) onboard NASA's TIMED satellite, finally enables unprecedented time-dependent quantification of the neutral O density distribution in the upper thermosphere from the sensitive Arecibo ISR measurements of ion-neutral coupling. In this work, I will describe these recent advances in measurement capability and present initial results which reveal its powerful implications for advancing understanding of thermosphere composition variability.