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Quantification of the Vertical Transport and Escape of Atomic Hydrogen in the Terrestrial Upper Atmosphere
Author(s) -
Joshi P. P.,
Phal Y. D.,
Waldrop L. S.
Publication year - 2019
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2019ja027057
Subject(s) - atmospheric escape , thermosphere , physics , atmospheric sciences , flux (metallurgy) , atmosphere (unit) , earth radius , outflow , mesosphere , radius , thermal , radiative transfer , geophysics , computational physics , environmental science , ionosphere , astrophysics , planet , meteorology , materials science , plasma , stratosphere , magnetosphere , computer security , quantum mechanics , computer science , metallurgy
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.