In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects.
It is often seen as a bluish (or other color) glow in the air adjacent to pointed metal conductors carrying high voltages. Spontaneous corona discharges are undesirable where they waste power in high-voltage systems or where the high chemical activity in a corona discharge creates objectionable or hazardous compounds, such as ozone. Controlled corona discharges are used in a variety of filtration, printing and other processes.