Quantification of the Vertical Transport and Escape of Atomic Hydrogen in the Terrestrial Upper Atmosphere

Abstract

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.

Coupling From the Middle Atmosphere to the Exobase: Dynamical Disturbance Effects on Light Chemical Species

“…In the upper mesosphere and above, the chemical lifetime of H becomes comparable to or longer than dynamical time scales, implying that horizontal and vertical transport play an important role in determining its variability and spatial distribution (Brasseur & Solomon, 2005). For example, eddy and molecular diffusive processes, as well as dynamical vertical transport, strongly influence the amount of H that flows vertically from the mesopause region through the exobase (Joshi et al, 2019; Kockarts, 1972); this is the process through which Earth loses its water (Catling & Kasting, 2017; Hunten & Strobel, 1974; Liu & Donahue, 1974b).…”

The Diffusion Limit of Photoevaporation in Primordial Planetary Atmospheres

“…Newly formed planets with primordial atmospheres efficiently absorb X-ray and ultraviolet (XUV) photons, triggering photoevaporation and the gradual loss of their hydrogen reservoirs. Observations suggest that atmospheric evaporation is prevalent, as evidenced by detections of leaking hydrogen (Joshi et al 2019) and xenon isotopic ratios on Earth (Porcelli & Pepin 2014) and by exoplanet population trends, such as the bimodal radial distribution and sub-Jovian desert (Fulton et al 2017). The physics of XUV-induced mass loss is less clear, however, with two major models being suggested: inviscid hydrodynamic outflow (Tian et al 2005) and diffusion-limited escape (Zahnle et al 2019).…”

Solar Cycle, Seasonal, and Dawn‐To‐Dusk Variations of the Hydrogen in the Upper Thermosphere

“…Since the 1980s, ground‐based observations of the Balmer‐α (656.3 nm) emission has been used to monitor the long‐term variations of the exospheric hydrogen (e.g., Nossal et al., 1993, 2004, 2008, 2019). In the last two decades, the geocoronal Lyman‐α (121.6 nm) emission has also been routinely observed by the Global Ultraviolet Imager (GUVI) onboard NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (e.g., Joshi et al., 2019; Paxton et al., 2017; Qin et al., 2017; Qin & Waldrop, 2016) and by the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) missions (e.g., Cucho‐Padin & Waldrop, 2018, 2019, 2020; Zoennchen et al., 2010, 2013, 2015). Despite decades of observations, the atomic hydrogen remains one of the least‐understood atmospheric constituents, in that discrepancies between models and observations have long been reported in the literature (e.g., Bishop et al., 2001; Gallant et al., 2019; Nossal et al., 2012; Qin & Waldrop, 2016; Waldrop & Paxton, 2013).…”

“…Ultraviolet Imager (GUVI) onboard NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (e.g., Joshi et al, 2019;Paxton et al, 2017;Qin et al, 2017;Qin & Waldrop, 2016) and by the Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) missions (e.g., Cucho-Padin & Waldrop, 2018, 2020Zoennchen et al, 2010Zoennchen et al, , 2013Zoennchen et al, , 2015. Despite decades of observations, the atomic hydrogen remains one of the least-understood atmospheric constituents, in that discrepancies between models and observations have long been reported in the literature (e.g., Bishop et al, 2001;Gallant et al, 2019;Nossal et al, 2012;Qin & Waldrop, 2016;Waldrop & Paxton, 2013).…”