by Isaac Asimov
The fading of radio signals generally occurred at night. In 1926, Appleton found that, shortly before dawn, radio waves were not reflected back by the Kennelly-Heaviside layer but were reflected from still higher layers (now sometimes called the Appleton layers), which begin at a height of 140 miles (figure 5.3).
Figure 5.3. Profile of the atmosphere. The jagged lines indicate the reflection of radio signals from the Kennelly-Heaviside and Appleton layers of the ionosphere. Air density decreases with height and is expressed in percentages of barometric pressure at sea level.
For all these discoveries Appleton received the Nobel Prize in physics in 1947. He had defined the important region of the atmosphere called the ionosphere, a word introduced in 1930 by the Scottish physicist Robert Alexander Watson-Watt. It includes the later-named mesosphere and thermosphere and is now divided into layers. From the stratopause up to 65 miles or so is the D region. Above that is the Kennelly-Heaviside layer, called the D layer. Above the D layer, to a height of 140 miles, is the E region—an intermediate area relatively poor in ions. This is followed by the Appleton layers: the F1 layer at 140 miles and the F2 layer at 200 miles. The F1 layer is the richest in ions, the F2 layer being significantly strong only in the daytime. Above these layers is the F region.
These layers reflect and absorb only the long radio waves used in ordinary radio broadcasts. The shorter waves, such as those used in television, pass through, for the most part. Hence, television broadcasting is limited in range—a limitation that can be remedied by satellite relay stations in the sky, which allow live television to span oceans and continents. The radio waves from space (for example, from radio stars) also pass through the ionosphere, fortunately; if they did not, there would be no radio astronomy possible from Earth’s surface.
The ionosphere is strongest at the end of the day, after the day-long effect of the sun’s radiation, and weakens by dawn because many ions and electrons have recombined. Storms on the sun, intensifying the streams of particles and high-energy radiation sent to the earth, cause the ionized layers to strengthen and thicken. The regions above the ionosphere also flare up into auroral displays. During these electric storms long-distance transmission of radio waves on the earth is disrupted and sometimes blacked out altogether.
It has turned but that the ionosphere is only one of the belts of radiation surrounding the earth. Outside the atmosphere, in what used to be considered “empty” space, satellites in 1958 disclosed a startling surprise. To understand it, we must make an excursion into the subject of magnetism.
Magnets
Magnets got their name from the ancient Greek town of Magnesia, near which the first lodestones were discovered. The lodestone is an iron oxide with natural magnetic properties. Tradition has it that Thales of Miletus, about 550 B.C., was the first philosopher to describe it.
MAGNETISM AND ELECTRICITY
Magnets became something more than a curiosity when it was discovered a steel needle stroked by a lodestone was magnetized and that, if the needle was allowed to pivot freely in a horizontal plane, it would end up lying approximately along a north-south line. Such a needle was, of course, of tremendous use to mariners; in fact, it became indispensable to ocean navigation, though the Polynesians did manage to cross the Pacific from island to island without a compass.
It is not known who first put such a magnetized needle on a pivot and enclosed it in a box to make a compass. The Chinese are supposed to have done it first and passed it on to the Arabs, who, in turn, passed it on to the Europeans. This is all very doubtful and may be only legend. At any rate, in twelfth century the compass came into use in Europe and was described in detail in 1269 by a French scholar best known by his Latinized name of Peter Peregrinus. Peregrinus named the end of the magnet that pointed north the north pole and the other the south pole.
Naturally, people speculated about why a magnetized needle should point north. Because magnets were known to attract other magnets, some thought there was a gigantic lodestone mountain in the far north toward which the needle strained. (Such a mountain is used to great effect in the tale of Sinbad the Sailor, in The Arabian Nights.) Others were even more romantic and gave magnets a “soul” and a kind of life.
The scientific study of magnets began with William Gilbert, the court physician of Queen Elizabeth 1. It was Gilbert who discovered that the earth itself is a giant magnet. He mounted a magnetized needle so that it could pivot freely in a vertical direction (a dip needle), and its north pole then dipped toward the ground (magnetic dip). Using a spherical lodestone as a model of the earth, he found that the needle behaved in the same way when it was placed over the northern hemisphere of his sphere. Gilbert published these findings in 1600 in a classic book entitled De Magnete.
For a long time, scientists speculated that the earth might have a gigantic iron magnet as its core. Although the earth was indeed found to have an iron core, it is now certain that this core cannot be a magnet, because iron, when heated, loses its strong magnetic properties (ferromagnetism, the prefix coming from the Latin word for “iron”) at 760° C, and the temperature of the earth’s core must be at least 1000° C.
The temperature at which a substance loses its magnetism is called the Curie temperature, since it was first discovered by Pierre Curie in 1895. Cobalt and nickel, which resemble iron closely in many respects, are also ferromagnetic. The Curie temperature for nickel is 356° C; for cobalt, it is 1075° C. At low temperatures, certain other metals are ferromagnetic: below −188° C, dysprosium is ferromagnetic, for instance.
In general, magnetism is a property of the atom itself; but in most materials, the tiny atomic magnets are oriented in random directions, so that most of the effect is canceled out. Even so, weak magnetic properties are often evidenced, and the result is paramagnetism. The strength of magnetism is expressed in terms of permeability. The permeability of a vacuum is 1.00 and that of paramagnetic substances is between 1.00 and 1.01.
Ferromagnetic substances have much higher permeabilities. Nickel has a permeability of 40; cobalt, of 55; and iron, in the thousands. In such substances, the existence of domains was postulated in 1907 by the French physicist Pierre Weiss. These are tiny areas, about 0.001 to 0.1 centimeters in diameter (which have actually been detected), within which the atomic magnets are so lined up as to reinforce one another, producing strong, overall fields. In ordinary non magnetized iron, the domains themselves arc randomly oriented and cancel one another’s effect. When the domains are brought into line by the action of another magnet, the iron is magnetized. The reorientation of domains during magnetism actually produces clicking and hissing noises that can be detected by suitable amplification, termed the Barkhausen effect after its discoverer, the German physicist Heinrich Barkhausen.
In antiferromagnetic substances, such as manganese, the domains also line up, but in alternate directions, so that most of the magnetism is canceled. Above a particular temperature, substances lose antiferromagnetism and become paramagnetic.
If the earth’s iron core is not itself a permanent magnet because it is above the Curie temperature, then there must be some other way of explaining the earth’s ability to affect a compass needle. What that way might be grew out of the work of the English scientist Michael Faraday, who discovered the connection between magnetism and electricity.
In the 1820s, Faraday started with an experiment that had been first described by Peter Peregrinus (and which still amuses young students of physics). The experiment consists in sprinkling fine iron filings on a piece of paper above a magnet and gently tapping the paper. The shaken filings tend to line up along arcs from the north to the south poles of the magnet. Faraday decided that these marked actual magnetic lines of force, forming a magnetic field.
Faraday, who had been attracted to the subject of magnetism by the Danish physicist Hans Christian Oersted’s observation in 1820 that an electric current flowing in a wire deflected a nearby compass needle, carne to the conclusion that the current must set
up magnetic lines of force around the wire.
He was all the more convinced since the French physicist Andre Marie Ampère had gone to study current-carrying wires immediately after Oersted’s discovery. Ampère showed that two parallel wires with the current flowing in the same direction attracted each other; with currents flowing in opposite directions, they repelled each other. This was very like the fashion in which two magnetic north poles (or two magnetic south poles) repelled each other while a magnetic north pole attracted a magnetic south pole. Better still, Ampère showed that a cylindrical coil of wire with an electric current Rowing through it behaved like a bar magnet. In memory of his work, the unit of intensity of electric current was officially named the ampere in 1881.
But if all this were so, thought Faraday (who had one of the most efficient intuitions in the history of science), and if electricity can set up a magnetic field so like the real thing that current-carrying wires can act like magnets, should not the reverse be true? Ought not a magnet produce a current of electricity that would be just like the current produced by chemical batteries?
In 1831, Faraday performed the experiment that was to change human history. He wound a coil of wire around one segment of an iron ring and a second coil of wire around another segment of the ring. Then he connected the first coil to a battery. His reasoning was that if he sent a current through the first coil, it would create magnetic lines of force that would be concentrated in the iron ring, and that this induced magnetism, in turn, would produce a current in the second coil. To detect that current, he connected the second coil to a galvanometer—an instrument for measuring electrical currents, which had been devised by the German physicist Johann Salomo Christoph Schweigger in 1820.
The experiment did not work as Faraday had expected. The flow of current in the first coil generated nothing in the second coil. But Faraday noticed that, at the moment when he turned on the current, the galvanometer needle kicked over briefly, and it did the same thing, but in the opposite direction, when he turned the current off. He guessed at once that it was the movement of magnetic lines of force across a wire, not the magnetism itself, that set up the current. When a current began to flow in the first coil, it initiated a magnetic field that, as it spread, cut across the second coil, setting up a momentary electric current there. Conversely, when the current from the battery was cut off, the collapsing lines of magnetic force again cut across the wire of the second coil, causing a momentary surge of electricity in the direction opposite that of the first flow.
Thus, Faraday discovered the principle of electrical induction and created the first transformer. He proceeded to demonstrate the phenomenon more plainly by using a permanent magnet and moving it in and out of a coil of wire; although no source of electricity was involved, a current flowed in the coil whenever the magnet’s lines of force cut across the wire (figure 5.4).
Figure 5.4. A Faraday experiment on the induction of electricity. When the magnet is moved in or out of the coil of wire, the cutting of its lines of force by the wire produces an electrical current in the coil.
Faraday’s discoveries not only led directly to the creation of the dynamo for generating electricity but also laid the foundation for James Clerk Maxwell’s electromagnetic theory, which linked light and other forms of radiation (such as radio) in a single family of electromagnetic radiations.
EARTH’S MAGNETIC FIELD
Now the close connection between magnetism and electricity points to a possible explanation of the earth’s magnetism. The compass needle has traced out its magnetic lines of force, which run from the north magnetic pole, located off northern Canada, to the south magnetic pole, located at the rim of Antarctica, each being about 15 degrees of latitude from the geographic poles. (The earth’s magnetic field has been detected at great heights by rockets carrying magnetometers.) The new suggestion is that the earth’s magnetism may originate in the flow of electric currents deep in its interior.
The physicist Walter Maurice Elsasser has proposed that the rotation of the earth sets up slow eddies in the molten iron core, circling west to east. These eddies have the effect of producing an electric current, likewise circling west to east. Just as Faraday’s coil of wire produced magnetic lines of force within the coil, so the circling electric current does in the earth’s core. It therefore creates the equivalent of an internal magnet extending north and south. This magnet, in turn, accounts for the earth’s general magnetic field, oriented roughly along the axis of rotation, so that the magnetic poles are near the north and south geographic poles (figure 5.5).
Figure 5.5. Elsasser’s theory of the generation of the earth’s magnetic field. Movements of material in the molten nickel-iron core set up electric currents which, in turn, generate magnetic lines of force. The dotted lines show the earth’s magnetic field.
The sun also has a general magnetic field, which is two or three times as intense as that of the earth, and local fields, apparently associated with the sunspots, which are thousands of times as intense. Studies of these fields (made possible by the fact that intense magnetism affects the wavelength of the light emitted) suggest that there are circular flows of electric charge within the sun.
There are, in fact, many puzzling features concerning sunspots, which may be answered once the causes of magnetic fields on an astronomic scale are worked out. In the course of a sunspot cycle, the spots appear only at certain latitudes, and these latitudes shift as the cycle progresses. The spots show a certain magnetic orientation that reverses itself in each new cycle, so that the total cycle from maximum at one magnetic orientation to maximum at the same magnetic orientation is about 21 years long, on the average. The reasons for this sunspot activity are still unknown.
We need not go to the sun for mysteries in connection with magnetic fields. There are problems here on earth. For instance, why do the magnetic poles not coincide with the geographic poles? The north magnetic pole is about 1,000 miles from the North Pole. Similarly, the south magnetic pole is about 1,000 from the South Pole. Furthermore, the magnetic poles are not directly opposite each other on the globe. A line through the earth connecting them (the magnetic axis) does not pass through its center.
Again, the deviation of the compass needle from true north (that is, the direction of the North Pole) varies irregularly as one travels east or west. In fact, the compass needle shifted on Columbus’s first voyage—a circumstance Columbus hid from his crew lest they become terrified and force him to turn back.
This is one of the reasons the use of a magnetic compass to determine direction is less than perfect. In 1911, a nonmagnetic method for indicating direction was introduced by the American inventor Elmer Ambrose Sperry. It takes advantage of the tendency of a rapidly turning heavy-rimmed wheel (a gyroscope, first studied by the same Foucault who had demonstrated the rotation of the earth) to resist changes in its plan of rotation. This tendency can be used to serve as a gyroscopic compass, which will maintain its reference to a fixed direction and serve to guide ships or rockets.
But if the magnetic compass is less than perfect, it has been useful enough to serve human beings for centuries. The deviation of the magnetic needle from the true north can be allowed for. A century after Columbus, in 1581, the Englishman Robert Norman prepared the first map indicating the actual direction marked out by a compass needle (magnetic declination) in various parts of the world. Lines connecting those points on the planet that show equal declinations (isogonic lines) run crookedly from north magnetic pole to south magnetic pole.
Unfortunately, such maps must be periodically changed, for even at one spot the magnetic declination changes with time. For instance, the declination at London shifted 32 degrees of arc in two centuries; it was 8 degrees east of north in 1600 and steadily swung around counterclockwise until it was 24 degrees west of north in 1800. Since then, it has shifted back and, in 1950, was only 8 degrees west of north.
Magnetic dip also changes slowly with time for any given spot on Earth, and the ma
p showing lines of equal dip (isoclinic lines) must also be constantly revised. Moreover, the intensity of Earth’s magnetic field increases with latitude and is three times as strong near the magnetic poles as in the equatorial regions. This intensity also changes constantly, so that maps showing isodynamic lines must also be periodically revised.
Like everything else about the magnetic field, the overall intensity of the field changes. For some time now, the intensity has been diminishing. The field has lost 15 percent of its total strength since 1670; if this loss continues, the intensity will reach zero by about the year 4000. What then? Will it continue decreasing, in the sense that it will reverse with the north magnetic pole in Antarctica and the south magnetic pole in the Arctic? In other words, does Earth’s magnetic field periodically diminish, reverse, intensify, diminish, reverse, and so on?
One way of telling whether it indeed can is to study volcanic rocks. When lava cools, the crystals form in alignment with the magnetic field. As long ago as 1906, the French physicist Bernard Brunhes noted that some rocks were magnetized in the direction opposite to Earth’s present magnetic field. This finding was largely ignored at the time, since it did not seem to make sense; but there is no denying it now. The telltale rocks inform us that not only has Earth’s magnetic field reversed, it has done so many times: nine times in the last 4 million years, at irregular intervals.
The most spectacular finding in this respect is on the ocean floor. If melted rock is indeed pushing up through the Global Rift and spreading out, then as one moves east or west from the Rift, one comes across rock that has solidified a progressively longer time ago. By studying the magnetic alignment, one can indeed find reversals occurring in strips, progressively farther from the Rift, at intervals of anywhere from 50,000 to 20 million years with the pattern on one side of the Rift being the mirror-image of that on the other. The only rational explanation so far is to suppose that there is sea-floor spreading, and there are magnetic-field reversals.
