system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo/Elysium] What stops the atmosphere from escaping out the top and sides of the Halo/space station?
gpt
The same electromagnetic field that shields the rings from solar radiation.

Field generators withing the ring structure creates an electromagnetic field that keeps the solar wind out but also keeps the polar molecules of the atmosphere in. Combine this with the artificial gravity created by the spinning of the ring and the atmosphere stays intact.