Secondly, What is the effect of noctilucent clouds on on our atmosphere? Is it clear? Could they be protecting us/earth from some solar radiation? Or are they having the opposite effect (increasing the greenhouse-effect, and/or reflecting more solar radiation into our atmosphere)?
Idea: Nanoparticles (or other charged dust) blasted beyond the stratosphere that we can control! Keep it where it can block the sun, occasionally produce festive light shows, and keep the alien's from reading our collective thoughts!