Corona discharge
From HvWiki
Also known as St. Elmo's Fire. A beautiful eerie blue glow that appears on high voltage conductors when the electric field is high enough to ionize the air. From Latin: corona- a crown- since it often occurs in little jets that look like the points of a crown.
High voltage generators (such as a Tesla coil) can give off corona, streamers, sparks, or any combination of them, depending on the voltage, power, and frequency of the output.
