A corona discharge starts when a free electron is accelerated by an electric field and impacts on a Nitrogen molecule with enough energy to knock 2 or more electron out of their orbits, ionizing the molecules. The liberated electrons result in an electron avalanche.
When the electrons colliding with the molecule no longer have enough energy to knock an electron out of its orbital part, the electrons energy is absorbed resulting in an excited molecule. The excited molecule will return to its ground state after emission of the excess energy as photons. This is called fluorescence.
In the case of Nitrogen, the emitted photons are mainly in the UV range (100 – 400nm). Other air molecules may also fluoresce, adding their light.
The UV emission is shown below, overlaid onto the natural sunlight spectrum. The visible range is from 400nm to 700nm.
The fluorescence spectrum of air is so faint that even after 10 000x amplification it barely peaks above the natural sunlight spectrum.
Below 300nm the sunlight spectrum drops to near zero intensity, this is due to the fact that the O-Zone layer absorbs these frequencies. The air fluorescence spectrum continues down to ~240nm.
UV below 300nm (UVc) is generated by a few sources: