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:
The lateral transport of helium in the exosphere impacts the distribution of helium in the upper thermosphere, and thermospheric circulation of helium affects its density at the exobase. A physics-based coupled thermosphere exosphere model, ExoTherM, has been developed which is novel in its ability to allow each of the two regions to influence the other. This work describes the output of this model in the low-Earth orbit region with a focus on the implications of neutral mass density and composition for satellite drag. A comparison is made to the empirical model NRLMSISE-00 for solar minimum conditions during June solstice. The location where the dominant gas species changes from oxygen to helium helps to establish the density response of the atmosphere in the low-Earth orbit region to solar energy and corresponds to changes in gas-surface interactions between the atmosphere and spacecraft. Percent differences in neutral mass density between the two models maximizes near the helium to oxygen transition (e.g., 600 km during solar minimum). While the compositional balance between helium and oxygen number density is consistent between the two models in the winter hemisphere, the helium to oxygen transition occurs more than 150 km higher in altitude, on average, at high latitudes in the summer hemisphere in ExoTherM. A hypothetical constellation of Starlink satellites is found to experience an effective mass density that is consistently higher in ExoTherm and has a lower average helium to oxygen ratio.
Date of Conference: September 16-19, 2025
Track: Atmospherics/Space Weather