Limit particle movement to emitter shape

I have a sphere shaped particle emitter, with a radius of 15 and a radius thickness of 0.1. Essentially the emitter is a thin shell of a sphere. This correctly limits particle emission points to that shell, but they move outward (away from the sphere's center).

I'd like them to move along the surface of the sphere, ideally limited to within that shell. I've tried using the "Velocity over Lifetime" radial controls but that doesn't quite achieve what I'm looking for. Is there any way to do this with the standard particle editor?

Thanks :)