Using WACCM-X neutral densities for orbital propagation: challenges and solutions

Atmospheric drag is a major perturbation in Low Earth Orbit (LEO). The neutral density obtained from atmospheric models is a major source of uncertainty in drag calculations and therefore orbital propagation in LEO. Many atmospheric models are available, with fast empirical models most commonly used. We explore the challenges and benefits of using numerical models, specifically the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) as part of the Community Earth System Model (CESM). Solutions are presented to overcome the challenges of using numerical models for neutral densities, along with a unique dataset covering the historic space age.

WACCM-X is a numerical model, solving 3D momentum, energy and continuity equations. This allows it to resolve smaller scale features than seen in the High Accuracy Satellite Drag Model (HASDM) or empirical models such as the Naval Research Laboratory’s Mass Spectrometer Incoherent Scatter radar (NRLMSIS) model. As WACCM-X includes chemistry and physics, it can also model the long-term density decreases caused by thermospheric contraction due to carbon dioxide emissions, or short-term density changes caused by large-scale events such as volcanic eruptions, or small-scale density increases caused by Joule heating.

There are challenges in using WACCM-X for neutral density modelling. The model’s upper boundary changes in altitude between around 400 to 700 km, primarily due to solar activity conditions. Therefore, density output is not available in a large proportion of LEO. To address this, WACCM-X has been extended to higher altitudes with the use of diffusive profiles (as used in Jacchia and MSIS models) using the upper boundary as a reference point. WACCM-X neglects helium, which contributes over 50% of the global-mean neutral mass density above 600 km during low solar activity. The winter helium bulge also accounts for over 80% of neutral mass density at high latitudes above 500 km in the winter hemisphere, which is currently missed by WACCM-X. Two methods of adding helium as a minor species to the model post-run have been investigated. The helium profile can be obtained by scaling NRLMSISE-00 output to fit WACCM-X and using the resultant NRLMSISE-00 helium profile, or by using a separate, uncoupled helium model. The NRLMSISE-00 method is suggested for already obtained output (variables allowing), while the uncoupled helium model is suggested for new simulations.

To investigate long-term changes and model specific historic events, the Space Environment and Radio Engineering (SERENE) group has run WACCM-X from the year 1950 to the present day, obtaining key variables such as neutral density and winds at half-hourly cadence on a 1.9 x 2.5-degree latitude by longitude grid with quarter scale height vertical resolution. It includes the changing chemistry associated with carbon dioxide and other greenhouse gas emissions, the ionospheric impacts of Earth’s changing magnetic field, and much more. Along with the above solutions, this provides a unique dataset of the thermosphere within LEO through the entire space age.