Kopp-Etchells Effect

Source: U.S. Army/ Sgt. Machael J. MacLeod

Do you see the halos around the helicopter? I saw this and thought it looked cool. What is actually happening is a matter of debate. What is known is that these halos tend to form around the helicopter blades when helicopters kick up dust in the desert and they are visible at night. The light intensity is greatest at the blade edges.

Some say that these halos are the result of discharges of static electricity. When the dust hits the nickel-titanium alloy that is used to protect the helicopter propellers, static electricity develops. This electricity discharges, causing the glow. The problem with this theory is that, if this were happening, the halos would have a blue tint, when they really have a red tint.

Some say that the dust particles are being heated by the collisions to glow red-hot. Similar to a meteor, the particles would burn up. The color of the light seems reasonable with this explanation (I have not seen a measurement of the light spectrum). The problem with this theory is that, if this were the case, the propellers would heat up, as well, which clearly is not happening since the propeller would have serious problems at the temperatures at which that might happen.

Another theory (put forward by physics blogger Kyle Hill; I find it the most convincing of those that I have come across) is that the dust hits the propellers and knocks off tiny pieces of the nickel-titanium alloy. This alloy is what is called pyrophoric. This means that it can randomly ignite in air. Another pyrophoric material is steel, which, when hit with flint, generates a spark. The friction between the dust particles and the blades would ignite the particle. You may be concerned that we use steel everywhere. The steel in your parent's cars does not randomly catch fire. There is no need to worry. Steel is most likely to ignite in a powdered form.

What do you think?
References: NPR, The Daily Mail, Kyle Hill