Non-thermal hydrogen atoms in the terrestrial upper thermosphere

Abstract

Model predictions of the distribution and dynamical transport of hydrogen atoms in the terrestrial atmosphere have long-standing discrepancies with ultraviolet remote sensing measurements, indicating likely deficiencies in conventional theories regarding this crucial atmospheric constituent. Here we report the existence of non-thermal hydrogen atoms that are much hotter than the ambient oxygen atoms in the upper thermosphere. Analysis of satellite measurements indicates that the upper thermospheric hydrogen temperature, more precisely the mean kinetic energy of the atomic hydrogen population, increases significantly with declining solar activity, contrary to contemporary understanding of thermospheric behaviour. The existence of hot hydrogen atoms in the upper thermosphere, which is the key to reconciling model predictions and observations, is likely a consequence of low atomic oxygen density leading to incomplete collisional thermalization of the hydrogen population following its kinetic energization through interactions with hot atomic or ionized constituents in the ionosphere, plasmasphere or magnetosphere.

Neutralized solar wind ahead of the Earth's magnetopause as contribution to non-thermal exospheric hydrogen

“…In this article, we have discussed a non-thermal hydrogen component originating from charge-exchange interactions of exobasic terrestrial hydrogen atoms with protons of the shocked solar wind plasma ahead of the magnetopause. Though this H component definitely represents a contribution to non-thermal hydrogen in the terrestrial exosphere with a relative relevance increasing with height, it has also become clear from the calculations presented in this article that this contribution cannot explain the observations made by Qin and Waldrop (2016). Concerning the Lymanalpha resonance glow, the trans-magnetopause contribution discussed in this article due to its rather low relative density in heights below three Earth radii can only become recognizable at large heights of about five to eight Earth radii, while Qin and Waldrop (2016) claim to clearly identify the influence of a non-thermal H component from their glow measurements already at heights of 3000 km.…”

“…Though this H component definitely represents a contribution to non-thermal hydrogen in the terrestrial exosphere with a relative relevance increasing with height, it has also become clear from the calculations presented in this article that this contribution cannot explain the observations made by Qin and Waldrop (2016). Concerning the Lymanalpha resonance glow, the trans-magnetopause contribution discussed in this article due to its rather low relative density in heights below three Earth radii can only become recognizable at large heights of about five to eight Earth radii, while Qin and Waldrop (2016) claim to clearly identify the influence of a non-thermal H component from their glow measurements already at heights of 3000 km. Thus, an idea, alternative to ours presented here and bringing in suprathermal H atoms at lower heights, might be to study whether the observationally indicated non-thermal H component could perhaps be due to a non-thermalized form of H atoms originating in the upper thermosphere via photo-dissociation of H 2 O through OH producing photo-dissociative H atoms with velocities of about 8 km s −1 (e.g., see Keller, 1990), which as non-thermalized H products represent a hydrogen component with an effective temperature of about 4000 K.…”

“…At the end of this article, we once again want to refer back to the beginning where we looked at the problem raised by Qin and Waldrop (2016) who had found indications for the existence of a non-thermal exospheric hydrogen component in their Lyman-alpha glow observations with the satellite TIMED/GUVI, a component for which no explanation as yet is available. In this article, we have discussed a non-thermal hydrogen component originating from charge-exchange interactions of exobasic terrestrial hydrogen atoms with protons of the shocked solar wind plasma ahead of the magnetopause.…”

“…This, in fact, has the effect to reduce the effective exobasic hydrogen temperature with respect to the exobasic oxygen temperature and thus reduces the effective H-atom escape flux (see Fahr, 1976). Nevertheless, these reasons for H-atom deviations from local exobasic thermodynamic equilibrium conditions do not explain what surprisingly and most recently has been observed by Qin and Waldrop (2016). These authors find that the upper exospheric hydrogen density distribution, especially during solar minimum conditions, is substantially higher than can be understood as a result of a terrestrial H-exosphere connected with exobasic oxygen temperatures (see Hodges, 1994).…”

“…In a most recent paper by Qin and Waldrop (2016), it had been found that the scale height of hydrogen in the upper exosphere of the Earth, especially during solar minimum conditions, appears to be surprisingly large. This indicates that during minimum conditions when exobasic temperatures should be small, large exospheric H-scale heights predominate.…”