There's an article on lightning in the May '05 Scientific American. It seems that the generation of X-rays by lightning was a surprise to researchers. I would be surprised if lightning did not generate X-rays.

New observations of x-rays from lightning bolts support the hypthesis that lightning somehow accelerates electrons to nearly the speed of light in a phenomenon called runaway breakdown.

....(RREA) Relativistic Runaway Electron Avalanche - According to this scenario, the runaway electrons themselves generate more energetic seed electrons by bumping forcefully into air molecules and knocking off other high-energy electrons. These knocked-off electrons then run away and collide with more air molecules, producing still more enegetic seed electrons, and so on. The result is a large avalanche of high-energy electrons that grows exponentially with time and distance.....

The Brief Life of Lightning

1. A fast moving proton from space collides with an air molecule (usually nitrogen or oxygen) in the upper atmosphere, producing a shower of high-energy particles.

2. Particles in the shower, including enegetic electrons, hit air molecules in a thundercloud, ejecting other high-energy electrons. Accelerated by electric fields stretching between regions of negative and positive charge, the particles initiate an avalanche of runaway electrons, which generate gamma rays as they blast through the cloud. This runaway breakdown may serve as the catalyst for lightning.

3. Once lightning is initiated, the electrons carve an ionized channel called the stepped leader. At each step, electrons accumulate at the leader's tip, creating an intense localized field that accelerates more runaway electrons. The particles collide with air molecules, producing bursts of x-rays. The process repeats until the stepped leader, which can diverge into branches, reaches the ground.

4. Once the leader connects to the ground, a large pulse of current flows through the channel. The current heats the air up to 30,000 degrees celsius, causing the flash of visible radiation called the return stroke.

sorry, I just noticed you followed my last remark on this thread. The 'runaway' electron avalanche can happen wherever electric fields are greater than the breakeven field over a distance longer than the characteristic length of 'runaway' avalanche e-folding (on the order of 10 meters). On earth, thunderstorms are pretty much the only such environment.

also, I just received my NewScientist mag that covered the subject. I noticed they conflated two separate but related phenomenon as the same one: x-ray generation in leaders due to acceleration of cold electrons to runaway energies and lightning initiation due to cosmic-ray shower initiated massive runaway electron avalanches (which also produce x-rays). The problem is that they advertise it all as 'lightning initiation', which is misleading. In fact, some of the quotes of the researchers were false in stating that lightning leader 'stepping' is not well understood and that it might be a product of runaway breakdown. It is quite well understood, and has nothing to do with runaway avalanches (contrary to the claims made in the article). It is simply due to the well-documented 'pilot' process of bipolar streamer generation at the front of the leader streamer-zone. Runaway avalanching cannot possibly be responsible for stepping because the e-folding length of runaway avalanching (length required for 1 runaway electron to precipitate a net of e=2.7 runaway electrons) is simply too long compared to step lengths.

it is the formation of a bipolar streamer in open air; it happens in the region just ahead of the leader that is characterized by the boundary of the streamer-zone plasma. This boundary has both high electric fields and copious negative ions ready to lend free electrons for kick-starting a streamer

What is the maximum energy density that can occur in lightning? (in joules per cubic meter)