I got this from Dayton-Granger the manufacturers of static dissipating systems for aircraft and helicopters. The sparks you see at night result from an energy exchange between the airborne particle and the blade. The energy exchange manifests itself as a small spark. The electrical energy from the collective sparks builds up on the blade surface as static electricity. If this static charge can not drain off to the airframe and be dissipated via static wicks then the charge will build to a point where it will bleed into the air as a corona discharge which manifests itself as a glow following the tip path plane. This makes it very difficult to hide a helicopter at night in the desert.
