[Astromancer]

I don't see the value of a pro-psi force in the Ring Islands. Just assume the anti-psi force is A) subtle on Earth propper, it prevents those with less than a certain amount of skill learning psi (which means skilled Psions can act on Earth) and B) the anti-psi force is unequally distributed on Yrth.

I might suggest that Yrth's moon causes the anti-psi force to wane when Yrth's moon is brighter. And that each orbit of Yrth's moon around Yrth slightly weakens the force. This would cover the slow reduction of the No Manna area in the Banestorm book.

By the way, don't have the slow weakening actually be connected to the moonlight. Correlation isn't causation after all. But let the PCs think it might be. It's a fun red herring.