Did Dark Lightning turn out to be a real thing? Is this what produces gamma ray flashes from lightning storms?

Question

The Science at NASA video ScienceCasts: Dark Lightning describes observation of gamma rays from Earth by the Fermi Gamma-ray Space Telescope.

These gamma rays are associated with weather that produces lightning, but the actual mechanism for gamma ray production by weather was still uncertain at the time.

The video explains that a discharge mechanism called "Dark Lightning" might produce the gamma rays without producing the same visible light signatures that normal lightning produces.

Question: Did Dark Lightning turn out to be a real thing? Is this what produces gamma ray flashes from lightning storms?

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Screen shot from the video shows a graphic of a booklet with the following words on its cover:

AGU Fall Meeting, San Francisco, 3-7 December 2012

Answer 1

According to the American Geophysical Union and reported at phys.org:

Dark lightning is a burst of gamma-rays produced during thunderstorms by extremely fast moving electrons colliding with air molecules. Researchers refer to such a burst as a terrestrial gamma-ray flash (TGF).

However, things are not quite so simple. There are actually several gamma ray phenomena related to thunderstorms.

According to Gamma-ray glow preceding downward terrestrial gamma-ray flash:

Two types of high-energy events have been detected from thunderstorms. One is “terrestrial gamma-ray flashes” (TGFs), sub-millisecond emissions coinciding with lightning discharges. The other is minute-lasting “gamma-ray glows”. Although both phenomena are thought to originate from relativistic runaway electron avalanches in strong electric fields, the connection between them is not well understood.

The general thought is that electrons are accelerated to high speeds by the extreme electric field present around lighting but that's not yet a completely satisfying explanation.

Image from The Quest to Understand how Thunderstorms Create Gamma Rays

Capture of photonuclear neutrons is also a potential source of terrestrial gamma-ray flashes:

These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere. Terrestrial Gamma-Ray Flashes as Powerful Particle Accelerators)

TGF Afterglows: A New Radiation Mechanism From Thunderstorms considers three different mechanisms at different time scales.

• "TGFs originate from cold runaway where thermal electrons accelerate to tens of MeV in the strong electric fields of a propagating leader discharge."
• "Glows ... would originate from relativistic runaway electron avalanches (RREA) with feedback of photons and positrons creating new avalanches..."
• "TGF afterglow ... when photons in the TGF are energetic enough to release neutrons from air molecules by a photonuclear reaction.... During thermalization they can be captured again by nuclei and sometimes with the release of a high-energy photon...."