• Choose a category
• All
• Classic-Fiction

By Root 335 0

of these is the ionosphere, a region starting about 50 miles above the surface, where atmospheric gases are so thin that free electrons can exist for brief periods before being captured, or ionized, by free-ranging positive ions, which in turn are produced by solar X-rays and ultraviolet radiation. The ionosphere is organized, more or less, into a series of broad bands, or levels, which for convenience sake but with scant regard for felicity are unimaginatively called D, E, F, and Topside. The D region, the lowest, and Topside, which is pretty obviously the highest, have little impact on human life; both are regions of weak ionization. It is the middle two layers, E and F, that impact us most. E is sometimes known as the Kennelly-Heaviside layer (or just the Heaviside layer), after the American engineer Arthur Kennelly and the British physicist Oliver Heaviside. It is a layer of strongly ionized gas between 54 and 90 miles thick, and because of its frequency (eight hertz), it has the useful effect of reflecting medium-wave radio transmissions, thereby allowing Midwestern country-and-western stations to be heard over the horizon in, say, Maine, to the great benefit of listeners there pining for hurtin' music; I remember in the old Soviet Union how teenagers would use the effect to listen to the Beatles singing "Back in the USSR" on the BBC, to the great irritation of their politically correct elders. Its usefulness varies, though, by time of day, season, and sunspot activity, so radio stations are wise not to make extravagant promises. The F region, above 90 miles, does the same thing.

But apart from its impact on radio broadcasting, why pay it any attention? Because the ionosphere is a flexible, dynamic, and rather fragile system buffeted about by electromagnetic emissions, by variations in the earth's magnetic field, and by the acoustic motion of the atmosphere itself, which means it is acutely sensitive to atmospheric changes. To monitor the ionosphere, then, is a good way of keeping a watch on atmospheric events. But there's more: Many scientists believe that, as NASA puts it on its Web site, "there is persuasive evidence of an ionospheric precursor to large earthquakes," and that it could even be used as a predictor. This is partly because acoustic waves are generated both before and after earthquakes, but also because it is thought that part of the run-up to an earthquake is the generation of electromagnetic emissions, which have been detected in the ionosphere up to six days prior to a large quake. In other words, if we learn how, we might be able to use the ionosphere as an early-warning system, dramatically increasing the time people living in earthquake zones have to react. A few days' warning could have made a massive difference, say, in the death toll of the Asian tsunami the day after Christmas 2004.

Above the ionosphere is the so-called magnetosphere. It is simple to visualize but complicated in its effects (which include, curiously, huge numbers of conspiracy-based Web pages that accuse the scientific community and the U.S. government in particular of massive cover-ups and fraud regarding its existence).

The magnetosphere is the field of influence of Earth's magnetic force—Earth is the equivalent of a normal, if rather over-size, bar magnet, mostly because of its nickel-iron core. Its most obvious use to us, of course, is in the magnetic compass; sailors have navigated by compass since it was independently invented sometime both in China and in Europe in the twelfth century, when savants in both places noted that a piece of lodestone (magnetic ore) floating on a stick in a jar of water pointed to the polestar. The magnetosphere ranges thousands of miles out into space—many orbiting space probes have as one of their scientific missions to help map this field, a task still incomplete. This simple and orderly system is complicated by the massive amounts of solar plasma making up the solar wind, which is itself strongly magnetized and pulses off the sun as a stream of high-energy electrons and protons at the