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The Magnetic Field and Exosphere of the Earth

space, geomagnetic, variations, time and sciences

THE MAGNETIC FIELD AND EXOSPHERE OF THE EARTH The examples cited would very likely suffice to illustrate the relations which are now being found between the Earth sciences and the sciences describing the universe surrounding the Earth. These relations between the sciences became established as we began to use our increased opportunities for the study of outer space to solve individual important problems. We wish to present an additional example, however, one which will show particularly clearly that this convergence of the interests of the different sciences simply represents objectively the natural relations which exist between the Earth and the space surrounding it.

Long ago navigators learned to guide their vessels according to a compass, the magnetic needle of which undeviatingly indicates the direction of the meridian. Moreover, it has been known for a long time that the Earth is a magnetized sphere and that the position of the magnetic pole is only slightly different from the position of the geographic pole. It was discovered just recently, however, that the Sun and the stars also possess magnetic fields, and that the stellar, solar, and geomagnetic fields vary continuously. Since paleomagnetism and its geological aspects were recently examined in an article by Kropotkin /9/, it will not be necessary to discuss the subject in any detail here. Let us just recall the following two well-established facts: 1) the polarity of the geomagnetic field changes approximately once during a period of several tens of thousands of years; 2) the magnetic poles drift over the surface of the Earth with the passage of geological time /10/. The second of these facts is related to the shifting of the Earth's crust relative to the rotation axis and does not have any significant effects in the space around the Earth. The reversal of the polarity of the geomagnetic field, however, and the disappearance of the field at the time when the field strength passes through zero, bring about considerable changes in the exosphere of the Earth, as the very distant portion of the terrestrial gaseous envelope is now called.

Studies made using artificial satellites and space rockets have shown that the atmosphere of our planet actually extends far out into space. In 1950 the Soviet astronomer Astapovich noted the presence of a gaseous tail near the Earth, oriented in a direction away from the Sun, and he estimated its extension as being several thousand kilometers. Then, in 1958, on the basis of satellite and rocket observations of cosmic rays, the Soviet physicist Vernov and the American scientist Van Allen discovered an intense radiation belt around the Earth, located at an altitude of about 2000 or 3000 km /11/. This belt is saturated with high-energy elementary par ticles. These charged nuclear particles, which are ejected by the Sun and the stars, fall into magnetic traps associated with the geomagnetic field. A cloud of very fine meteoric bodies is apparently picked up by these magnetic traps around the Earth as well, the electrical charges on the surfaces of the particles being important for such small particle masses (see § I of this article).

Thus, the radiation belts, and very likely the cloud of cosmic dust around the Earth as well, owe their presence to the extent to which the geomagnetic field can hold their constituent particles in the traps created by it. As the polarity of the geomagnetic field goes through its gradual

cycle of reversals, the magnetic traps of the Earth must become some times stronger and sometimes weaker, and they will disappear periodically for short periods of time (perhaps for some decades or centuries). The disappearance of the magnetic traps, even for a short time, destroys the radiation belts and the meteoric cloud around the Earth during this period. Such a pulsation of the exosphere, and the periodically recurring disappear ance of its basic elements, cannot take place without having some effect on the interaction between the Earth and other bodies in space. It is difficult to say at present just what effect these variations have on processes taking place in the Earth's atmosphere, but still it is possible to speculate on the nature of this effect. Most likely, when the barrier impeding the arrival of cosmic rays disappears, the ionization of the upper layers of the atmosphere will increase. At the same time, the reduction of the density of cosmic dust around the Earth will lead to a reduction of the absorption of solar radiation in interplanetary space. Atmospheric processes which are induced by ionization of the upper atmospheric layers will be intensified. For example, under these circumstances there will be a more intensive circulation of matter in the atmosphere, and climatic processes will become more pronounced. It is possible, therefore, that some long-period climatic variations depend on the variations in the geo magnetic field. These climatic changes must affect the entire Earth simultaneously, and in the past they must have greatly influenced animal and plant life on our planet.

In our brief account of the interdependence of terrestrial and space phenomena associated with the geomagnetic field, we began with the compass and ended with the possibility of climatic and biological variations which are effected simultaneously with variations in the Earth's magnetic field. However, many scientists now believe that the source of the geomagnetic field and its variations is the Earth's core, with its system of ring currents. Consequently, variations taking place within the very depths of the Earth are able to bring about changes in the distant exosphere of our planet and thereby to influence conditions on the Earth's surface.

The purpose of this article has been twofold. First of all, we have tried to emphasize that the individual Earth sciences and space sciences (such as geology, geophysics, geochemistry, astrophysics, and cosmogony) are now making more and more contact with one another. Secondly, we have tried to show that this trend toward a greater complexity and interrelation of fields which were previously relatively separate is a result of certain basic connections existing in nature, the recognition of which has now become imperative for modern science. If the reader has become aware of this through the examples presented in this article, then our goal has been achieved.