Is upper-atmospheric lightning loud?

Question

The upper layer of the atmosphere has much less density, and pressure. Do phenomena like blue jets, or gnomes even make a sound?

Answer 1

To hear means, someone needs an ear sensitive enough. As you pointed out, density is the problem. Single atoms or molecules might carry a lot of energy, but there are so few, that your ear might not react.

By the way, the same is true for temperature. It can be very hot in space (between galaxies even millions of K), but you would never feel it, because the frequency of the very fast (and therefore very hot) particles hitting you is so small that almost no energy is transfered.

But to come back to your question: I unfortunately don't have an altitude until that a human ear can here, but there is a discussion that might be interesting for further reading: https://space.stackexchange.com/questions/46427/till-what-altitude-above-earth-sounds-can-be-heard