Very interesting. Thanks Tom. Revelations such as this always make me wonder how many other mysterious unseen and unkowns there are that we can't even imagine exist.
The space Shuttle and Space Station had reported the lightning sprites as visibly observed above the clouds, it would seem that these are the results of the strongest of lightning strikes that produce the sprites. It would seem reasonable that the dark lightning may result from the sprites shooting out into the upper atmosphere.
As a firefighter for many years, I have seen many of the damaging results of lightning strikes where structures, power lines and equipment and trees were struck. The power of nature is truly amazing...
These are phenomena that have been there all the time and we are just becoming aware of them. Now that we are studying them, more is revealed and that makes it all the neater.
I think it was NOVA that had a show on them a year or two ago.
Here's an article from WaPo that complements the original post.
The only difference between X-rays and gamma rays is their energies. When I was a pup, X-rays came from electron orbitals and gamma rays from atomic nuclei, but that has changed. Physics was simpler then. The Van Allen Belts hadn't been discovered. Same with lasers.