Dielectric breakdown

This is an eerie light source, I think anyway. It is ordinary plastic insulated wire in a twisted pair, but I am applying a high frequency high voltage. There are no connections at the end. If the voltage was DC nothing would happen, although the insulation would be stressed beyond its designed spec.
This is due to dielectric breakdown in the air-plastic interface. The potential difference is sufficient to cause breakdown in the air so a localised discharge is occurring along the touching surfaces between the wires. There is UV emission form the nitrogen in the air and that is fluorescing the paper below the wire. There is also ozone production, so between the UV, ozone and electrical stress the insulation on the cable will fail given an extended period of time.