Plasma generation in a gap around a sliding contact

It has been hypothesized that plasma is generated at a sliding contact. However, it has not yet been found. Here, we report on a discovery of plasma generated in the microscopic gap around a sliding contact, having an elliptical shape with a horseshoe pattern and with a size beyond a hundred micrometers. It emits mostly invisible ultraviolet (UV) photons and, to a lesser extent, infrared (IR) photons. It must be a main source of the curious tribophysical and chemical phenomena. The origin, characteristics and the relation to these curious phenomena are discussed.