Why do high-voltage transmission lines emit green fluorescence at night?

What you see is the corona phenomenon. Generally speaking, corona will appear on EHV lines, especially when the air is humid. Simply speaking, high-voltage lines discharge air, but in a complicated way, it is. . . . . . . . . .

Corona is a local self-sustaining discharge of gas medium in uneven electric field. The most common form of gas discharge. Near the tip electrode with large radius of curvature, the local electric field strength exceeds the ionization field strength of the gas, which ionizes and excites the gas, thus corona discharge occurs. When corona occurs, visible light around the electrode is accompanied by hissing. Corona discharge can be a relatively stable discharge form, or it can be an early development stage in the process of gap breakdown in uneven electric field.

The formation mechanism of corona discharge varies with the polarity of tip electrode, which is mainly caused by the different accumulation and distribution of space charges during corona discharge. Under the action of DC voltage, both negative corona and positive corona gather space charges near the tip electrode. In negative corona, when electrons cause collision ionization, they are driven to the space far away from the tip electrode and form negative ions, while positive ions gather near the electrode surface. When the electric field continues to strengthen, positive ions are sucked into the electrode, and pulse corona current appears at this time, and negative ions diffuse into the gap space. After that, repeat the next ionization and charged particle movement process. This cycle produces corona current in the form of many pulses. Corona current was discovered by G W Trichel in 1938, which is called Trichel pulse. If the voltage continues to rise, the pulse frequency and amplitude of corona current increase and turn into negative glow discharge. When the voltage rises again, negative streamer discharge appears, which is also called feather discharge or brush discharge because of its shape. When the negative streamer discharge continues to develop to the counter electrode, it leads to spark discharge and the whole gap is broken down. Positive corona also distributes positive ions near the tip electrode, but they are constantly repelled to the gap space, and electrons are sucked into the electrode, which also forms repeated pulse corona current. When the voltage continues to rise, streamer discharge appears, which may lead to gap breakdown.

I hope my answer can help you!