Monday, July 18, 2005

Extraterrestrial activity on earth

Anyone who pictures scientists as calm and reserved should have seen us when the data from the gamma-ray flash appeared in our computer. One might have thought that our favorite team had just scored the winning touchdown at the superbowl.

That was one of the best articles in Scientific American I've ever read. The topic is lightning. As it turns out, we know almost nothing regarding how lightning works! Of course, Benjamin Franklin proved 250 years ago that it is an electric effect, with electrons accumulating in the clouds and a current (spark) produced with the positive ions in the surface.

However, none knows how the charge gets there in the first place. On top of that, all the measurements of storms show that the electric field inside the clouds is 10 times smaller than the electric field that causes the small spark after we walk on a carpet and then touch some metal item. How then can the lightning form in the first place?

The incident described at the beginning of the thread occurred when they discovered that lightnings produce gamma-rays and x-rays, which proved that an effect called runaway breakdown occurs. What probably happens is that high energy cosmic rays from outer space hit some air molecule (oxygen, nitrogen) and cause an electron to be removed (yes, the origin of the lightning is extraterrestrial!). The high speed electron causes an avalanche, as it keeps bouncing on more and more molecules and more and more electrons accumulate, while producing gamma-rays.. These electrons are accelerated even more to speeds close to the speed of light from the local electric fields inside the clouds. This is the catalyst of lightning.

Once the lightning has initiated, the first electrons start heading downwards. They follow a bumpy left-right discrete step motion as they collide with air molecules, ionizing them, produce x-rays while changing direction. Eventually these first leader electrons reach the ground.

Once this ionized channel has formed, a large current (more electrons!) flows through the channel to the ground. These electrons move so fast that 1) heat up the air, producing the visible part of the lightning and 2) the heat compresses and decompresses the air violently, causing the audible part, the thunder.