Helicopter rotors are fitted with abrasion shields along their leading edges to protect the blades. These abrasion strips are often made of titanium, stainless steel, or nickel alloys, which are very hard, but not as hard as sand. When a helicopter flies low to the ground in desert environments, sand can strike the metal abrasion strip and cause erosion, which produces a visible corona or halo around the rotor blades. The effect is caused by the pyrophoric oxidation of the ablated metal particles. In this way, the Kopp–Etchells effect is similar to the sparks made by a grinder, which are also due to pyrophoricity.
Sand particles are more likely to hit the rotor when the rotorcraft is near the ground. This occurs because sand is blown into the air by the downwash and then carried to the top of the rotor disk by a vortex of air. This process is called recirculation and can lead to a complete brownout in severe situations. The Kopp–Etchells effect, however, is not necessarily associated with takeoff and landing operations. It has been observed without night vision goggles at altitudes as high as 1700 ft.https://en.wikipedia.org/wiki/Kopp%E2%80%93Etchells_effect