A corona is a process by which a current, develops from an electrode with a high potential in air by ionizing that fluid so as to create a plasma around the edges of current carrying wire or electrode. The ions generated pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules.Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. On application of strong electromagnetic field neutral gas is converted to ionized molecule due to breaking of molecular bonds, thus turning it into a plasma, which contains charged particles: positive ions and negative electrons.
When the potential gradient i.e. direction towards maximum value, is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the air around that point will be at a much higher gradient than elsewhere as charge is inversely proportional to area. Air near the point can become ionized i.e. conductive, while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor because now the net area of charge has increased and hence reduces its sharpness. Since the new conductive region is less sharp, the ionization may not extend further. Outside the region of high potential gradient , the charged particles slowly find their way to an oppositely charged object and are neutralized as these charge will induce opposite charge.
The current carried in the power line alternates direction, usually 60 times a second (60Hz). This propagates to the electrical field, affecting the plasma, and producing the audible BUZZING vibration of air.
If the situation is such that the ionized region continues to grow instead of stopping at a certain radius, a completely conductive path may be formed, between two sharp points , resulting in a momentary or long duration CORONA discharge.