Why high-voltage power lines glow with a faint, invisible, and energy-wasting blue light

Nature · Published 3 October 2026 Swipe more like this →

In short: A corona discharge is an electrical leak that turns air into glowing plasma. It happens when high voltage ionizes surrounding gas, creating a chain reaction of electron avalanches. While often an unwanted energy drain for power utilities, this same phenomenon is harnessed for everything from air filtration to industrial photocopying.

A corona discharge occurs when the electric field surrounding a conductor exceeds the dielectric strength of the surrounding fluid, typically air. This creates a localized region of ionized gas, or plasma, where charge continuously leaks from the conductor. Unlike an electric arc, which forms a low-resistance path between two conductors, a corona is a single-electrode discharge. It persists because the geometry prevents the ionized region from bridging the gap to an oppositely charged surface, keeping the discharge contained near the high-voltage source.

The process relies on electron avalanches. A stray event, such as a cosmic ray, ionizes a neutral atom, creating a free electron and a positive ion. The high electric field accelerates the electron, which then strikes other atoms, knocking out more electrons in a self-sustaining chain reaction. The characteristic glow is produced when electrons recombine with ions, releasing photons. Because this is a non-thermal plasma, the gas remains near ambient temperature, unlike the intense heat of an electric arc.

The intensity of the electric field is highest at sharp points or small-diameter wires, which is why corona discharge often begins at these locations. At sea level, the critical field strength is roughly 30 kV/cm. Engineers frequently use rounded electrodes or corona rings to spread the field and suppress these discharges. This is vital because coronas produce corrosive gases like ozone and nitric oxide, which can embrittle materials and damage sensitive electrical components like transformers and capacitors.

Despite these drawbacks, controlled coronas are highly useful. They are employed in electrostatic precipitators to scrub pollutants from air, in mass spectrometers for sample ionization, and in ionic wind devices. The asymmetry between positive and negative coronas—driven by the mass difference between electrons and ions—means they behave differently; for instance, a negative corona generates significantly more ozone than a positive one, a factor that must be managed in industrial applications.

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