I'm imagining it's something related to the copper wire.

I always wanted to replicate this with a helium balloon and a long, wet string coated with copper filings.

You'd probably need a very large balloon to overcome the weight of the string

Maybe just salt water and skip the filings?

Even better, use hydrogen!

Hydrogen string is notoriously hard to tie knots in.

Copper emits a green-blue light in the flame test https://www.youtube.com/watch?v=dwsexjcROH4

The surrounding air becomes ionized. Since Earth’s atmosphere is mostly nitrogen, and ionized nitrogen often emits a purple glow.

Why don't we do this on the norm and somehow harness the energy?

https://en.wikipedia.org/wiki/Harvesting_lightning_energy

Prize is too low. Says it only delivers 38 gallons of gas equivalent energy, assuming no loss which there will be.

There are some places that are hit by lighting with high regularity where it might make sense. Lightning rods on tall buildings. Somewhere near here: https://en.wikipedia.org/wiki/Catatumbo_lightning

Lighting rods also work by dissipating the local electric potential and reducing the likelihood of a lightning strike. That's why they are pointed, or fractal in the newer ones.

From https://en.wikipedia.org/wiki/Lightning_rod "and that a considerable electric current can be measured through the conductors as ionization occurs at the point when an electric field is present, such as happens when thunderclouds are overhead."

You can get a ground charge even when thunderclouds aren't present. It just has to be windy. I've always wondered if it was worth collecting this, say with a field with a grid of interconnected lightning rods in a windy location.

It must have been so fun to be Benjamin Franklin. He installed an electric bell in-line on his first lightning rods so that this increase in electrical potential would cause the bell to ring, signaling that the danger for lightning was increased.

You can experience this voltage potential firsthand: take a friend to a hill or somewhere where high‑voltage power lines are nearer to the ground. One person, wearing rubber-soled shoes, stands tall with an arm in the air. The other squats down with both hands on the ground. The standing person uses the non-raised hand to gently touch the sensitive skin of the other person (ear lobe, lips), and they can feel an electric arc.