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THEY have been mistaken for extraterrestrial visitors, supernatural phenomenon and everything in between. And recently, the crew of an Air UK jet, which was struck by lightning, saw "fireballs in the cabin". Ball lightning is the term that is given to these mysterious balls of fire that usually appear during or after a thunderstorm. After many years of mystery, the phenomenon has finally been explained, thanks to Antonio Ranada, physicist at the Complutense University in Madrid, Spain.
For many years, scientists dismissed ball lightning as myth or an optical illusion. They reasoned that a sphere of glowing ionised gas, or plasma, should explode almost instantly, even if it is momentarily contained by the magnetic field of a lightning strike (During thunderstorms, frequent lightning magnetically charge the surrounding area, creating strong magnetic fields). "Calculations suggest that it should blow up in a fraction of a second," says Ranada.
Then, in 1969, a respected radio astronomer wrote an article in Nature about his encounter with a fireball that passed through the walls of a jetliner. Eyewitnesses did not feel heat radiating from it, but its touch could burn. People began to accept that the phenomenon actually existed, even if its bizarre behaviour defied its explanation.
Now Ranada and his colleagues believe they may have the answer. They say that linked magnetic loops might explain how fireballs could last so long. Lightning strikes normally create horizontal magnetic fields all around them, but vertical fields may sometimes form. And under certain circumstances, vertical and horizontal loops might link together and form a ball.
Glowing plasma could be trapped inside the magnetic lines. If there were just one magnetic field, it would expand with the plasma, and the plasma ball would explode instantly. But the two fields are constrained by the link between them - they and the plasma cannot expand. The glowing fireball effect lasts until the plasma cools. "It's perfectly clear that it can last for 10 to 15 seconds," says Ranada. In the cooling plasma, electrons become bound to their atoms. This increases electrical resistance, killing the current within the plasma and weakening the surrounding the magnetic fields.
This theory also explains why the fireball radiates little or no heat, yet its touch can burn. Most of the ball is cold, but along the field lines - the loops and a few streamers coming off the fireball - the temperatures each 16,000 c or even more.
Other scientists claim they have seen the magnetic loops that could precede ball lightning. Mark Rader, electrical engineer at the University of Tennessee, USA, has studied sparks that come off high-voltage generators. "We saw spark-overs that were looping back on themselves," he says.
If two loops formed so they were linked together, ball lightning might result. But Rader's loops have never linked, nor have they separated from the spark. "They were always connected to the lightning," he says. Since ball lightning always appears to be free-floating, Rader thinks that under certain conditions it might be possible for these precursor loops to detach themselves from the lightning and begin to float.