Two recent lightning studies provide interesting insight into the formation of lightning and the terrestrial gamma flashes, (also known as dark lightning), that sometimes accompany it. While both studies make use of the radio pulses created during lightning formation they seem to differ in their explanations of how the radio pulses are created.
|Lake Maracaibo from Wikipedia http://en.wikipedia.org/wiki/Catatumbo_lightning|
First a little background on lightning formation. The following great summary is from a recent post by +John Baez.
"Lightning happens in stages. First, a streamer of electricity travels from one charged area to another, say, from a cloud to the ground, or from one layer within a cloud to another. This prompts a return stroke with the reverse charge to go in the opposite direction. The initial streamer electrified the air it moved through, creating a path of least resistance that allows the return stroke to carry a much greater current."
Both of the studies indicate that radio pulses associated with lightning are emitted from the thundercloud prior to the actual lightning, during the streamer formation mentioned above. The two studies differ, however, in their attribution of the source of the radio pulses. I'm a bit perplexed by the whole thing. If there are any lightning experts that would like to chime in and clear everything up, that would be awesome!
Reference 1 details a study that was made possible by the coincidental detection of the same lightning flash over Lake Maracaibo, (picture 1), by two different satellites, (one of them was the satellite mentioned in ), as well as two different radio-based terrestrial lightning detectors. By sorting out the signals from the various detectors, they were able to determine that the radio pulse took place shortly after the terrestrial gamma flash, (TGF), and they attribute the creation of the radio pulse to the gamma flash.
"We find that the TGF was produced deep in the thundercloud at the initial stage of an intracloud (IC) lightning before the leader reached the cloud top and extended horizontally. A strong radio pulse was produced by the TGF itself."The timing information agrees with the second study. It's not made clear by the paper how the gamma flash created the radio pulse however.
The second study was based on lightning measurements made over Russia and Kazakhstan. This project was concerned with measuring the same radio pulses mentioned in  in an effort to determine what initiates lightning creation. Here's where we arrive at the GIGO model, (gammas in, gammas out). The researchers present a model based on their data that indicates that the initial breakdown of the lightning conduction channel is caused by a cosmic gamma ray creating an ionization cascade. They attribute the radio pulses, which they agree occur prior to the lightning, to the resulting high electron currents created by the ionization cascade.
Interestingly, the ionization cascade proposed by  for lightning formation is the same process used, (on a much smaller scale), in particle detectors known as wire chambers. A high energy particle, (such as a cosmic ray), strikes an electron orbiting an atom of the gas filling the detector. The electron is knocked out of its orbit ionizing the atom. An electric field maintained in the detector accelerates the now free electron and it in turn ionizes additional gas atoms. The electrons from these collisions ionize still more atoms and a cascade of electrons is formed which is detected by sensitive amplifiers built into the detector.
In particle detectors, if the cascade runs away and becomes too large it can damage the detector. Special care is taken to make sure no small particles of dust or other pollutants are present in the detector as these can serve as nucleation points for further cascades. The Russian study indicates that small water and ice particles have the same effect in thunderclouds that dust has in wire chambers. They serve as nucleation points that allow cosmic rays of lower energy than initially expected to create electron cascades that led to lightning strikes.
Thanks to +Hans Havermann for pointing out the terrestrial gamma flash study!
All thoughts, comments, suggestions and clarifications are always welcome!
1. Terrestrial Gamma Flash study
Østgaard N., Gjesteland T., Carlson B.E., Collier A.B., Cummer S., Lu G. & Christian H.J. (2013). Simultaneous observations of optical lightning and terrestrial gamma ray flash from space, Geophysical Research Letters, n/a-n/a. DOI: 10.1002/grl.50466
2. Russian Lightning study
Gurevich A.V. & Karashtin A.N. (2013). Runaway Breakdown and Hydrometeors in Lightning Initiation, Physical Review Letters, 110 (18) DOI: 10.1103/PhysRevLett.110.185005
3. +John Baez's post on dark lightning
4. Open access synopsis of the Russian lightning study from the +American Physical Society