Simulation Study on the Variability of Earth's Exosphere
 

Sang-Yun Lee 
Postdoctoral Researcher
Physics Department -The Catholic University of America 
NASA Goddard Space Flight Center

Wed, February 26, 2025 - 4:00 PM

This study presents a new kinetic simulation model focusing on exospheric hydrogen dynamics, tracing individual hydrogen particles from specific exospheric locations back to the exobase while incorporating Earth’s gravitational force, solar radiation pressure, and the Coriolis force induced by Earth’s orbit around the Sun. While gravitational force dominates near Earth, solar radiation pressure significantly alters particle trajectories in the anti-sunward direction—boosting the number density by up to five times—and the novel inclusion of the Coriolis force, under a constant exobase boundary, further increases the density by up to 6%, particularly as hydrogen atoms can travel for several hours, allowing time-varying exobase conditions to enhance this effect. Atomic hydrogen, the major constituent of Earth’s exosphere, exhibits a density distribution that is highly sensitive to solar activity: the number density and temperature at the exobase are determined by upper atmospheric heating and thermal evaporation, while solar radiation pressure from Lyman-alpha radiation shapes the geotail structure. To model a realistic distribution of exospheric hydrogen, the NRLMSIS-2 model is used as the inner boundary condition, and kinetic simulations were conducted over a 27-day period during both solar minimum and solar maximum under geomagnetically quiet conditions. During solar minimum, despite a nearly constant F10.7 index, the exospheric density at 2–3 Re is dynamic, with variations diminishing at greater distances; in contrast, during solar maximum, when F10.7 ranges from 96 to 208 and the Lyman-alpha flux varies by approximately 65%, the modeled density fluctuates by up to 35%—with stronger solar activity reducing density below 3 Re due to thermal evaporation and increasing it beyond 4 Re as upper atmospheric heating enables hydrogen atoms to reach higher altitudes—resulting in overall higher exospheric hydrogen density that aligns well with TWINS geocorona observations.

Refreshments served at 3:45 PM

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