The excited sodium then emits orange light. Now it is the excited FeO that emits the orange light. It is now also suspected where the ozone and oxygen come from. Many trains turn out to have a tubular structure, with two bands of intense emission where the line of sight crosses the edge of the tube. Subsequent modeling by John Plane of the University of East Anglia showed that the tubular structure can be explained if the oxygen atoms are created by the fireball, while the ozone is that in the ambient atmosphere.
Many fundamental questions remain.
The model does not yet explain why the trains are so dark in the center and why they expand linearly in time. It is clear that persistent trains can help understand the physical processes that lead to the natural airglow in the upper atmosphere by setting constraints on diffusion timescales and relevant chemistry mechanisms. To order your copy, see SETI Institute store .