Hello. I wonder if someone could answer a question on a particular aspect of lightning?

 

Why do positive streamers appear purple in colour?

 

Thank you

Ionisation energy of the gasses they pass through, if you look at the Spectra for Air in specific for Nitrogen, you`ll see it has Many bands in the Purple wavelength.

I don`t have the Spectral data for O2 and Argon to hand, but they may also be worth looking up, look around the 400 nm band.

Thanks!