The phenomenon of violet glow, and hissing noise accompanied by the production of ozone gas in an overhead transmission line is known as the corona effect.
Corona effect also known as corona discharge is an electrical discharge that is caused due to ionization of a fluid such as air surrounding a conductor carrying a high voltage. It signifies a local region where the air or some other fluid has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air.
When an alternating voltage is applied across two conductors for which the spacing is large in comparison to their diameters, condition of atmospheric air remains same and there is no apparent change in the surrounding until the applied voltage is low. If the applied voltage exceeds a certain value the conductors are surrounded by a faint violet glow called corona.
In technical terms the voltage above which corona occurs is termed as critical disruptive voltage.
The presence of cosmic rays, ultraviolet radiations and radioactivity in nature results in some ionization. During normal environmental conditions, air around the power transmission conductors comprises of ionized particles. When a potential difference between is applied between conductors, potential gradient sets up in air that has maximum value at conductor surface. Free electrons present under influence of potential gradient achieve greater velocities.
The velocity of free electrons is correlated to the applied voltage, the higher the voltage is, the greater potential gradient. If the potential gradient at the surface of conductor reaches 30 kV per cm, the velocity attained by the free electrons is creates a neutral molecule with enough force to dislodge one or more electrons from it. As a result, ion and one or more free electrons are created which in turn are accelerated until they collide with other neutral molecules, thereby producing other ions.
The process of ionization keeps on increasing and finally resulting in formation of corona or sparking between the conductors.
If transmission line conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors. If conductor is not polished, the rough points will appear brighter.
See below video on Coron Effect in Transmission lines: