Tuesday, May 03, 2005

Gamma Rays From Thunderstorms

One of the signs that Rusi Taleyarkhan's approach to sonofusion might be workable is that U. Illinois researchers were recently able to detect extremely high-energy photons coming from cavitating bubbles in a sulfuric acid solution. Tangientially related is this Duke University press release (hat tip: Science Blog) noting that gamma ray bursts have been unexpectedly observed coming from lightning strikes. The report says that the bursts occur about a millisecond before a lightning stroke, and at much lower elevations than expected if the trigger were cosmic rays, as had been anticipated.
The detailed Duke-led analysis also disclosed that, on average, [Terrestrial Gamma ray Flashes, or TGFs] occurred 1.24 milliseconds before their associated lightning strokes. "That was something we absolutely were not expecting," Cummer said. "But the coincidence between the lightning and the TGFs we found is too good to be random. So, even if the TGFs precede the lightning, they are in some way connected."

Their paper suggests one possibility for such a negative cause-and-effect relationship. Perhaps "TGFs are produced by a process associated with the development of the observed lightning stroke, but that actually occurs about 1 millisecond before the stroke itself," the authors wrote.

Nothing useful by itself of course, but perhaps it will turn into understanding of how plasmas operate, as gamma rays are usually indicative of nuclear processes at work, fission or fusion.

Update 5/5: More on this from New Scientist:

A few years ago, lightning researchers began to look for some other way the electrical breakdown of air might be triggered, and they have identified a chief suspect: cosmic rays. These are highly energetic particles that zip through space at close to the speed of light. Thousands of them bombard every square metre of Earth's atmosphere each second, many having travelled intergalactic distances. In 1992, Alex Gurevich of the P. N. Lebedev Physical Institute in Moscow had proposed a way that cosmic rays might seed lightning. When a cosmic ray strikes Earth's atmosphere, it could hit an air molecule, ionising it and producing an extremely energetic electron. In the electric fields near a storm cloud, such an electron could be accelerated to near the speed of light, then hit and ionise other air molecules, producing more and more electrons in a chain reaction. The ensuing avalanche of electrons would ionise the air, allowing charge to flow. Gurevich called the idea "runaway breakdown".

The theory was originally considered fairly maverick, but in the absence of other explanations, it is now becoming mainstream. The main advantage of runaway breakdown is that it requires a far smaller electric field to get started - around 300 kilovolts per metre, similar to that routinely measured in storm clouds. And electrons moving near the speed of light emit energetic radiation such as X-rays and gamma rays, providing a way to test the idea. In 2001, Charlie Moore and his colleagues at the New Mexico Institute of Mining and Technology in Socorro found the first direct evidence for runaway breakdown when they recorded X-rays shooting from the "leader" of a nearby lightning bolt. The leader is the nearly invisible path of current that moves towards the ground in halting steps, each between 50 and 100 metres long. Each step is separated by a 50 microsecond pause. Usually, the leader is negative, but as it approaches the ground, positive charges collect and are drawn upwards. When these two paths meet, the circuit between ground and sky is complete and lightning is unleashed.

Still, nobody has adequately explained how runaway breakdown is triggered; cosmic rays of the right energy (1016 eV) are too rare (once every 50s) to explain the earth's lightning activities. But Dwyer thinks he's on to something:
Dwyer would be very surprised to see the theory disproved now. "We know that runaway breakdown is occurring, and we know that lightning is occurring," he says. "Do these two things have anything to do with each other? That's just a question of being at the right place at the right time."
With, of course, the right instruments.