A MAGNET INSIDE THE EARTH?
When it became known that the interior of the earth is hot, it was obvious that the earth's magnetic field could not be a permanent magnet. This is because heating disorients various parts of a magnet destroying the ability to produce a coherent magnetic field. Thus arose the puzzle of where the earth's magnetic field comes from. Scientists now believe it to be caused by circulation of liquid metal in the outer core:
If friction can ionize the metal atoms, then the flow of ionized material becomes an electric current, which produces the magnetic field. Such a mechanism is known as a dynamo, a device that converts mechanical energy of motion into electrical energy. Thus the earth is more of an electromagnet than a permanent magnet.
I n appearance the magnetic field of the earth resembles that of a bar magnet inclined slightly to the earth's axis of rotation. The magnetic lines of force run between the north and south polar regions of the earth, much as the pattern formed by iron filings sprinkled around a bar magnet does. The intensity of the magnetic field decreases away from the earth's surface, but the magnetic field can still be measured many tens of thousands of kilometers out in space.
However it began, the earth's magnetic field has changed polarity (the north magnetic pole becomes the south magnetic pole and vice versa) many times over geologic time. Scientists trace the history of these changes by studying the magnetism frozen into rocks of different ages: I ron particles in molten lava beds align themselves along the lines of the existing magnetic force, and after the rocks solidify, they retain the orientation of the magnetic field indefinitely. Such rocks show that magnetic reversals have come at intervals as short as 35,000 years. Why the reversals? We do not know. One suggestion is that they may be related in some way to changes in the earth's rotation or in the fluid state of its outer core.
FAR MAGNETIC FiElDS
The magnetosphere is that part of the magnetic field surrounding the earth that exerts a force strong enough to control the motions of charged subatomic particles entering the field. Exerting a strong force even 50,000 kilometers away from the earth's surface, this magnetic field protects us from bombardment by many of the charged subatomic particles traveling through space at speeds that are a significant fraction of the speed of light.
From satellites monitoring the magnetic field we have learned much about the magnetosphere's strength, direction, and composition. It has several concentric zones; the pri ncipal ones are zones of high subatomic-particle densities known as the Van Allen radiation belts (named after American physicist James Van Allen, (1914- ) who discovered their existence in 1958 from Explorer satellite data). The Van Allen belts encircle the planet in two doughnutshaped regions about 3,000 and 17,000 kilometers from the earth's surface.
Charged particles, mainly protons and electrons, populate the magnetosphere's radiation belts. Most of these subatomic particles are ejected from the sun as a reasonably steady flow of matter in the plane of the ecliptic known as the solar wind. When the solarwind particles encounter the earth, they are either diverted away from it or trapped by its magnetic field.
The collision of solar-wind particles with the earth's magnetosphere creates a shock wave that distorts and compresses the magnetic field on the sunlit side and stretches it into a long tail on the night side. (A shock wave is a large-amplitude compression, such as the sonic boom made by a jet plane.)
INTERACTION OF MAGNETOSPHERE AND ATMOSPHERE
Charged particles constantly spread out of the outer Van Allen radiation belt and fall into the auroral latitudes of the earth's atmosphere. There they collide with atoms of oxygen and nitrogen and stimulate these gases to radiate pale greens and occasional bright reds in patches or across the whole sky. These are the auroras, called the northern lights in our hemisphere. They are most often seen in zones between 65° and 70° north and south magnetic latitudes. Because the subatomic particles enter the atmosphere easily when the solar wind is more intense, more auroras color our night skies during the height of the 11-year sunspot cycle, when the sun is emitting more subatomic particles.
COSMIC RAYS
In the early part of this century scientists found evidence for some kind of radiation entering the earth's atmosphere from outer space. Believing the radiation to be electromagnetic in nature, they called it "cosmic rays." Shortly thereafter cosmic rays were shown to consist of subatomic particles rather than electromagnetic waves; but the name stuck, and scientists still refer to them as rays. They should not be confused with very small meteoric particles that also enter the atmosphere.
Cosmic rays consist primarily of protons and helium nuclei (or alpha particles) with some heavier nuclei and a few electrons. They appear to be coming from all directions in space in about equal numbers. The kinetic energies of the cosmic-ray particles cover a very wide range, with the high-energy ones among the most energetic known in nature. There is a lowenergy-that is, a low-velocity-component of cosmic rays that we know to be coming from the sun. Thus scientists divided the cosmic rays into a Galactic cosmic-ray component coming from outside the solar system and a solar cosmic-ray component.
When cosmic-ray particles encounter the magnetosphere, the lowest-energy ones are trapped in the earth's magnetic field, while those with a little larger energies are channeled by the magnetic field to enter the earth's atmosphere at high magnetic latitudes (which is approximately the same as high geographic latitudes). The higher-energy cosmic rays from Galactic space penetrate the magnetosphere almost as if it were not there, striking the nuclei of atmospheric molecules, from which enormous showers of subatomic particles rain down on the surface of the earth. As we go about our daily lives, we are almost continually pierced by these cosmic-ray-shower particles.
The dynamics of these cosmic-ray showers and primary cosmic-ray particles have told us a lot about the earth's magnetic field and the magnetosphere. Charged particles, such as the cosmic-ray particles, are controlled in their motion by magnetic fields and thus are natural probes to reveal the intensity and direction of the magnetic field.
