Where is the top of the thermosphere? And why it matters.

The rapidly increasing population of active satellites and space debris in Low Earth Orbit (LEO) means that accurate and precise orbit prediction is becoming ever more important to avoid catastrophic collisions. Atmospheric drag is the second largest force on objects in LEO after gravity, and so orbit prediction requires models of the thermosphere that can predict the variations in density that directly affect atmospheric drag. The most popular physics-based global circulation models (GCMs) have chosen different upper boundary heights, ranging between 400-800 km for quiet-moderate activity levels. Yet orbit perturbations by atmospheric drag have been observed at much higher heights. How realistic is it to extrapolate densities above the boundaries of fluid models to altitudes that are notoriously poorly observed, and where particle trajectories are presumed ballistic? Furthermore, how well are we capturing the coupling of the ionosphere, magnetosphere and lower atmosphere? The thermosphere’s upper boundary is very susceptible to space weather and can rise by a few hundred km within a few hours in response to a sudden storm commencement and Joule heating, right into the path of a LEO satellite. Climate change is also causing the upper boundary to move down over long timescales, which is due to the cooling and contraction of the stratosphere, mesosphere and lower thermosphere in response to increasing CO2 levels.

We propose that one way to identify and estimate the top of the thermosphere is by monitoring objects in free-fall. We look at 38 Cubesats from the QB50 mission over their lifetime of 2017–2025, covering solar minimum and maximum; and at the whole catalogue of over 20,000 LEO satellites during the Gannon Superstorm of May 2024. In particular, we find that the “top of the thermosphere”, as evidenced by atmospheric drag, depends on the orbiting body, as well as space weather and climate change.