TL;DR, you need a magnetosphere to keep the solar wind from stripping your atmosphere. You could create an artificial magnetosphere by placing a really powerful magnet at the first Lagrange point such that the planet is shadowed by the object’s magnetotail.
This is a lot simpler than what I had conceived, rings of satellites orbiting the planet and doing… something that keeps the atmosphere stable. Of course there’s a lot more to maintaining a terraformed climate than just the magnetosphere, so maybe I’ll need those satellites after all.
If you want to get crazy, build a set of collectors to funnel the particles into a station, and make power either as a magnetohydrodynamic generator, or using it as fuel for fusion.
And what better giant magnet than an artificial sun? Obviously for bodies too far from the sun for the sun to provide meaningful illumination. The reflector bounces more light back at the planet and also prevents light pollution affecting nearby bodies so the sky isn’t full of artificial suns.
