TOEFL Reading練習 #028 — The Fireball That Refuses to Explain Itself(日本語解説)

The Fireball That Refuses to Explain Itself

¶1 Ball lightning is one of the oldest reported phenomena in atmospheric science and one of the least understood. It is a glowing sphere, usually the size of a grapefruit or a beach ball, that appears during thunderstorms, drifts through the air for several seconds, and then disappears, sometimes with a small explosion. Investigators have proposed several mechanisms to explain it, but the event is so rare and unpredictable that almost none of these proposals could be tested against real data. In 2012, a team of physicists recording an ordinary lightning storm captured, by pure chance, the first detailed measurements ever taken of a naturally occurring ball lightning. The result did not settle the debate; it complicated it.

¶2 Eyewitnesses describe ball lightning drifting at roughly walking pace, sometimes passing through closed windows or moving down the aisle of an aircraft cabin. It has been reported to last anywhere from a few seconds to nearly a minute before it fades quietly or ends with a pop. Because it cannot be triggered on demand or predicted in advance, laboratory tests can only imitate it. Most of the evidence collected over two centuries has been eyewitness testimony rather than instrument data. That changed on the evening of 23 July 2012. A team of physicists led by Jianyong Cen had set up slit spectrographs and cameras on the Qinghai Plateau to record ordinary cloud-to-ground lightning. About nine hundred meters from their equipment, a bolt struck the ground, and seconds later a glowing sphere rose from the strike point and drifted horizontally for roughly fifteen meters before fading out. For the first time, instruments rather than memory had captured a ball lightning event.

¶3 Two explanations have dominated the debate. One holds that ball lightning is a form of plasma, air ionized into a hot, charged gas briefly trapped by its own electric or magnetic field before the field collapses and the glow fades. A second, proposed by chemists John Abrahamson and James Dinniss in 2000, argues that a lightning strike can vaporize silicon-rich soil and eject a fine aerosol, a suspension of tiny solid particles in air. As these particles slowly oxidize, or combine with oxygen, they could sustain a glow far longer than a single spark. The two mechanisms predict different things about the light itself. A pure air plasma should glow mainly with the signature of nitrogen and oxygen. A burning aerosol of vaporized soil should also carry the signature of the elements in the ground. Spectroscopy, the technique of splitting light into its component wavelengths to identify which elements produced it, is the only way to tell the two apart, and 2012 was the first time anyone had applied it to a real event. The spectrum showed lines of silicon, iron, and calcium layered over the nitrogen and oxygen expected from ordinary air, closely matching the soil at the strike site.

¶4 The match seemed to confirm the aerosol theory, but two problems remain unresolved. [A] The brightness the team measured implied a total energy output higher than a slow-burning aerosol of that size should have been able to produce, given how much silicon a single lightning strike is thought to vaporize. [B] Either the strike vaporized far more material than existing models assume, or some additional process contributed to the light as well. [C] A second problem is harder to explain away. [D] Many reported sightings occur indoors, through closed windows, or inside aircraft cabins, far from any soil a lightning bolt could have struck, and the aerosol theory offers no mechanism for those cases at all.

¶5 The 2012 measurement remains the only detailed spectroscopic record of a natural ball lightning event, and despite decades of effort, no comparable data set exists to confirm or challenge it. Researchers have proposed that outdoor, soil-related sightings and indoor sightings might not be the same phenomenon at all, but rather two or more distinct effects that eyewitnesses have long grouped under a single name because they look alike. Testing that idea would require capturing another event under controlled conditions, and no one can predict when or where the next one will occur. More than sixty years after the first serious scientific attempts to explain it, physicists still cannot say with confidence whether they are chasing one mechanism or several.