Drag Coefficient Constraints for Space Weather Observations in the Upper
Thermosphere
Abstract
The space weather research community relies heavily on thermospheric
density data to understand long-term thermospheric variability,
construct assimilative, empirical, and semi-empirical global atmospheric
models, and validate model performance. One of the challenges in
resolving accurate thermospheric density datasets from satellite orbital
drag measurements is modeling appropriate physical aerodynamic drag
force coefficients. The drag coefficient may change throughout the
thermospheric environment due to model dependencies on composition and
altitude. As such, existing drag coefficient model errors may be
altitude and solar cycle dependent, with greater errors at higher
altitudes around 500 km near the oxygen-to-helium transition region.
This can lead to errors in orbit-derived density datasets and models. In
this paper, inter-satellite density comparisons at ~500
km are evaluated to constrain drag coefficient modeling assumptions.
Density consistency results indicate that drag coefficient models with
incomplete energy and momentum accommodation produce the most consistent
densities, while the standard diffuse modeling approach may not be
appropriate at these altitudes. Models with momentum accommodation
between 0.5 - 0.9 and energy accommodation between 0.83 - 0.96 may be
the most appropriate at upper thermospheric altitudes. Modeling drag
coefficients with diffuse gas-surface interactions could lead to errors
in derived density of ~25% and in-track satellite orbit
prediction uncertainty during solar maximum conditions on the order of
hundreds of meters.