zeeoo,
I have seen the tail rotor excite the particles at night, also. the photos I pulled off the net don't show it.
The effect I describe is actually also electric, in that the crystal glows because its electrons are disturbed and flow within it to make the light (and also some electricity, as I described in how a phonograph needle works).
Lu thinks the charge builds up faster than it is carried off, but that is dead wrong, because otherwise the way we ground helos for load pickup wouldn't work either. Lu can never drop his point, so I will stop debating this about now. It takes photos, a US Army web site that says "The glowing rings are made up of numerous small sparks resulting from grains of sand striking a normally-operating rotor blade", a guy scratching a crystal and Lu still prattles on about it being static electrcity. I can wait another 2 years for you to fnd a letter from a company that sells oat meal cookies, and they can help your case, perhaps.
Lu, I do not know what the hell a toroidal magnet on a Bell has to do with your mistaken belief that we see the sparkles on a rotor blade because the rotor is drinking Corona beer. Pray, don't tell me, let me remain forever uninformed!