Here is an explanation from Wikipedia: Abrasion strips on helicopter rotor blades are made of metal, often titanium<http://en.wikipedia.org/wiki/Titanium> or nickel<http://en.wikipedia.org/wiki/Nickel>, which are very hard, but less hard than sand. When a helicopter flies low to the ground in desert environments, sand striking the rotor blade can cause erosion. At night, sand hitting the metal abrasion strip causes a visible corona or halo around the rotor blades. The effect is caused by the pyrophoric<http://en.wikipedia.org/wiki/Pyrophoric> oxidation of eroded particles.
Wikipedia reference for above:
* Office of Naval Research Broad Agency Announcement(BAA): Advanced Helicopter Rotor Blade Erosion Protection<http://www.onr.navy.mil/02/baa/docs/BAA%2008-011_ONRBAA%2008-011.pdf> (pdf). United States Department of the Navy, Office of Naval Research. p. 3. BAA 08-011. Retrieved 2009-09-02. "An equally important problem with Ti protection is that a visible corona or halo is generated around the rotor blades at night from the sand impacting the Ti leading edge and causing Ti to spark and oxidize."