Q.What is corona discharge ?

Corona discharge is a self-sustained electrical breakdown of the air occurring only very close to a sharp point or edge of a highly charged conductor, where the field is intense enough to ionise the surrounding air.

1. The electric field near a sharp point/edge of a charged conductor is far stronger than near a flat/smooth surface (field is inversely related to the local radius of curvature).
2. When this field exceeds the breakdown/ionising field of air (about $3\times10^6$ V/m), the air molecules right around the point get ionised, producing a faint bluish/violet glow and a hissing sound.
3. This partial, localised discharge (unlike a full spark, which bridges the entire gap between two conductors) is called corona discharge; it causes gradual leakage of charge from the conductor and, along transmission lines, energy loss. …

Corona discharge is a self-sustained electrical breakdown of the air occurring only very close to a sharp point or edge of a highly charged conductor, where the field is intense enough to ionise the surrounding air.

1. The electric field near a sharp point/edge of a charged conductor is far stronger than near a flat/smooth surface (field is inversely related to the local radius of curvature).
2. When this field exceeds the breakdown/ionising field of air (about $3\times10^6$ V/m), the air molecules right around the point get ionised, producing a faint bluish/violet glow and a hissing sound.
3. This partial, localised discharge (unlike a full spark, which bridges the entire gap between two conductors) is called corona discharge; it causes gradual leakage of charge from the conductor and, along transmission lines, energy loss. …