Sarah Luettgen, University of Colorado Boulder; Eric Sutton, University of Colorado / SWx TREC; Jeffrey Thayer, University of Colorado
Keywords: thermosphere, exosphere, helium, modeling, drag, space weather, LEO, upper atmosphere, neutral density
Abstract:
At around 500km altitude is located the boundary between the thermosphere and exosphere, called the exobase. Here, too, is where the dominant atmospheric constituent begins to transition from oxygen to helium, impacting atmospheric drag predictions for Low Earth Orbit (LEO) objects. The location of the helium to oxygen (He/O) transition helps determine the mass density response of the thermosphere to solar input at LEO altitudes [1]. This transition is where the atmosphere experiences the greatest density perturbation during a geomagnetic storm [2]. Due to changes in gas-surface interactions for a helium or oxygen dominated atmosphere, it also coincides with increased errors in drag coefficient modeling and atmospheric density derived from accelerometers [3]. Finally, near and above this transition, helium density fluctuations have a significant impact on total mass density. Density and composition changes lead to uncertainty in satellite drag, which remains the greatest challenge to operators’ abilities to accurately assess collision risk and predict orbital trajectories [4]. The goal of this work is to better characterize factors that affect satellite drag in the LEO space environment, specifically total mass density and composition changes, with a physical thermosphere-exosphere.An understanding of the dynamics of the LEO space environment requires an understanding of helium dynamics. Helium is a light species and exospheric transport plays an important role in its composition near the exobase. To accurately represent the helium composition of the upper thermosphere, the effects of the exosphere must be considered. Conversely, to represent the helium population of the exosphere, accurate thermospheric boundary conditions must be supplied. However, no physical model capable of simulating the LEO region has captured the coupled thermosphere-exosphere feedback effect on helium density.Complicating this issue is the transition of physics governing atmospheric dynamics that occurs across the exobase. From the lower to mid thermosphere, the gas can be described using continuum or near-continuum mechanics. Atmospheric models in this region can use fluid dynamics to solve for the gas composition and motion. However, in the upper thermosphere and beyond, the gas is perturbed further and further from a Maxwellian velocity distribution. As the ratio of the mean free path of the gas to the scale height of the atmosphere increases, fluid approximations lose fidelity. Instead, single-particle dynamics methods such as direct simulation Monte Carlo (DSMC) can be used to describe the gas.We have coupled a fluid model of the thermosphere and a DSMC model of the exosphere such that each model provides a boundary condition for the other, creating a unified model across the exobase. The Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) is a fluid, time-dependent, global model that simulates the atmosphere from 32km altitude to roughly 500km altitude [5]. Currently, TIME-GCM’s upper boundary condition for helium is an analytical approximation for the vertical flux caused by exospheric transport at a static altitude of 500km, neglecting all collisions above the exobase [6,7,8]. However, this flux is applied at the upper boundary of TIME-GCM, which is variable in height. Furthermore, collisions do occur above the altitude at which it is applied. By simulating the transport of helium in the exosphere using single-particle dynamics, we can omit the assumptions made by this condition and instead directly model the ballistic motion in the lower exosphere. This can be accomplished by Monaco, a direct simulation Monte Carlo model capable of modeling rarefied gas dynamics [9]. Initial conditions for the atmosphere at the lower boundary of Monaco are supplied by TIME-GCM. This has resulted in a novel model, ExoTherM, whose thermospheric helium content responds to exospheric dynamics, and vis versa. This model spans from the mesosphere through the lower exosphere.The two-way coupled exosphere-thermosphere model will be used to investigate the impact of two-way physical modeling on the representation of composition and mass density in LEO. This study will focus on solstice conditions, when the winter high latitudes are inundated with helium while oxygen dominates in the summer high latitudes. The empirical model NRLMSISE-00 is widely used for determining upper atmospheric conditions (e.g., [10]) and will be used as a baseline with which to make a comparison.The outcomes of this study will include an examination of the dynamics of neutral mass density and the helium to oxygen ratio in the upper register of LEO in the context of its impact on LEO resident objects [11]. The upper register represents a region where the most populous LEO objects reside. Here, too, atmospheric drag and radiation pressure compete as the dominant nonconservative acceleration, gas‐surface interactions are more specular and less accommodating, and the gas composition transitions to predominantly helium and hydrogen. During solstice conditions, a change in the He/O ratio from less than 1 in the summer hemisphere to more than 50 in the winter hemisphere is possible within a single LEO orbit, leading to a change in drag coefficient. Poor representation of composition and temperature in the upper register, in addition to the changing conditions of gas‐surface interactions, can limit the accuracy of physically modeled drag coefficients and impact the prediction of an orbit’s trajectory. Comparing a physical model, ExoTherM, with an empirical model, NRLMSISE-00, reveals a neutral mass density discrepancy between the two that maximizes around 600km. Furthermore, the comparison reveals a difference of more than 150km in the altitude of the He to O transition in the summer hemisphere during solstice conditions. Both major discrepancies have implications for drag modeling and orbit prediction in the upper LEO register and raise questions about the way that NRLMSISE-00 models this region. By allowing for the lateral transport of helium to impact thermospheric parameters, ExoTherM presents a step forward in making physics-based models more physically representative of the regions they simulate.[1]Thayer et al. (2012) doi:10.1029/2012JA017832[2]Liu et al. (2014) doi:10.1002/2013JA019453[3]Bernstein and Pilinski (2022) doi:10.1029/2021SW002977[4]Mehta et al. (2023) doi:10.1016/j.asr.2022.05.064[5] Roble & Ridley (1994) doi:10.1029/93GL03391[6]Sutton et al. (2015) doi:10.1002/2015JA021223[7]Hodges and Johnson (1968) doi:10.1029/JA073i023p07307[8]Hodges (1973) doi:10.1029/JA078i031p07340[9] Dietrich & Boyd (1996) doi:10.1006/jcph.1996.0141[10] International Organization for Standardization (2022). ISO 14222:2022[11]Thayer et al. (2021) doi:10.1002/9781119815570.ch5
Date of Conference: September 16-19, 2025
Track: Atmospherics/Space Weather