What Is Corona Discharge?
Corona discharge is a type of electrical discharge that occurs when the electric field surrounding a conductor becomes strong enough to ionize the surrounding air — but not strong enough to cause a full arc or spark across to another conductor. The result is a faint, self-sustaining plasma region that glows with a soft blue or violet hue and produces a characteristic hissing sound.
The name "corona" comes from the Latin word for crown, describing the crown-like glow visible around high-voltage conductors under low-light conditions — an appearance that also inspired the naming of the Sun's outer atmosphere.
The Physics of Formation
Corona discharge begins when the electric field intensity at the surface of a conductor exceeds the dielectric breakdown strength of the surrounding gas (approximately 3 MV/m for dry air at sea level). This threshold is most easily reached at:
- Sharp edges and points — where electric field lines concentrate (the "point effect")
- Small-diameter conductors — thin wires have higher surface field gradients than thick ones
- High altitudes — lower air pressure reduces the breakdown threshold
- High humidity or pollution — contamination of conductor surfaces lowers the onset voltage
Once initiated, free electrons in the ionized region collide with neutral gas molecules, creating avalanche ionization. The discharge stabilizes into a steady glow rather than propagating into a full arc because the field strength drops off rapidly with distance from the conductor.
Types of Corona Discharge
The form corona takes depends on electrode geometry and polarity:
- Trichel Pulses: Repetitive micro-discharges on negative sharp points in air — highly regular and measurable
- Burst Corona: Irregular discharge on positive points at lower voltages