Magnetosphere

“ From the previous article we have come to the conclusion that not only is the earth a magnet, but that all the other planets are magnets also, so that if any of these are brought into the magnetic field of the sun, then the magnetic axis of the planet, which corresponds to the needle in our experiment, must assume a certain dip in relation to the sun, setting itself along those lines of force which are in the immediate neighbourhood of the planet. ”

“ Not only has the earth geographical North and South poles, but it has also magnetic North and South poles, and indeed has all the phenomena incidental to a magnet, such as magnetic dip and magnetic lines of force, as we shall see later on.
We know, however, that the earth is simply one of a system of planets, which revolves with all the others of that system round its central body the sun; and the question arises, whether the earth is the only one out of all the planets that is actually a magnet. Suppose it is affirmed that the earth is the only planet which is a magnet. ”

“ The changes in the magnetism of the earth and atmosphere give rise to these currents. They are developed in various forms. The following may be mentioned as the most important.
1st. The general and widespread perturbations, occurring within the body of the earth, and implicating immense areas, even whole continents.
2d. The interruptions of continuity of the solar currents as in the phenomena called sun-spots. These changes, to whatever cause due, are capable of disturbing the terrestrial magnetic equilibrium over varying areas, and of working instantaneously. ”

“ So as intensity increases, from the magnetic equator toward the poles, the needle, when so suspended as to permit of the motion, dips, inclines downward, and the dip is greatest, on the same parallel, where intensity is greatest. To my mind, the magnetic elements are very intelligible. They are all attributable to attraction, and attraction is greatest where intensity is greatest. There is nothing in the earth or atmosphere to make the needle point northerly rather than in any other direction, except magnetic intensity. ”

“ As we approached it in our air-ship, coming from the east, we could see, a hundred miles before we reached the continent, the radiance of its millions of magnetic lights, reflected on the sky, like the glare of a great conflagration. These lights are not fed, as in the old time, from electric dynamos, but the magnetism of the planet itself is harnessed for the use of man. ”

“ The earth itself, or the atmospheric envelope by which it is surrounded, is a most powerful magnet, and the lines of force which proceed whether from its interior, its solid shell, or vaporous covering, are held to be just such lines as those are which surround artificial magnets; proceed whence they may, they are supposed to extend through the atmosphere, and to reach even to the planetary spaces. ”

“ Surely, then, it cannot be considered as unphilosophical to inquire of magnetism for some of the anomalous movements that are observed in the atmosphere. These anomalies are many; they are not confined to the easting of the trade-winds ”

“ Must we seek the seat of these currents, that is to say, of the disturbed electricity, in the atmosphere, in the regions of planetary space, or in the polarity of the Sun and Moon? Galileo, in his celebrated 'Dialogo', was inclined to ascribe the parallel direction of the axis of the Earth to a magnetic point of attraction seated in universal space.
If we represent to ourselves the interior of the Earth as fused and undergoing an enormous pressure, and at a degree of temperature the amount of which we are unable to assign, p 190 we must renounce all idea of a magnetic nucleus of the Earth. ”

“ Her influence on terrestrial magnetism is now ascertained; the magnetism of the hemisphere that is turned toward the earth attracts the pole of our needles that is turned toward the south and increases the magnetism of our hemisphere; and as the magnetic, like the gravitating force, extends through space, the induction of the sun, moon and planets must occasion perpetual variations in the intensity of terrestrial magnetism, by the continual changes in their relative positions. ”

“ It seems probable also that the sun, because of its great mass and its rotation in the same direction as the earth's rotation, is a magnet, with polarities on the North and South sides of the equator, similar to terrestrial North and South magnetic poles.” Further, such a conclusion is entirely in harmony with the view of the solar system revealed in Art. 81, where we saw that each planet was an electrified body having its own electric field, with its lines of force, being capable of giving rise to all the phenomena associated with electricity. ”

“ The magnetic field is a force field. The Earth has a great magnetic field that seems to be related to the terrestrial core composition, a core of molten iron. This magnetic field acts like a protective shield, deflecting the high-speed particles coming from the Sun. ”

“ That half of the globe turned away from the sun will partake of the density of the ether at that distance, which is greater than on the side next the sun; the magnetic intensity ought, therefore, to be greater in the night than in the day. The poles of the great terrestrial magnets, or even the position of a magnetic needle on the surface, are continually placed by the earth’s rotation in a different relation to the axes of the terral vortex, and the tangential current, which is continually circulating around the globe, has its inclination to a given meridian in a perpetual state of change. ”

“ Thus then, when electrical discharges occur in the polar regions between the positive electricity of the atmosphere and the negative electricity of the earth, the magnetic poles of the earth would exercise a similar influence on the icy haze which is conceived essential to the evolution of the auroral light. Thus the arc seen by the observer would be that portion of the luminous ring above his horizon, varying with the distance from the pole. ”

“ We maintain, however, that the magnetick horizon is a plane level throughout touching the earth or a terrella in the place of some one region, with which plane the semi-diameter, whether of the earth or of the terrella, produced to the place of the region, makes right angles on every side. Such a plane is to be considered in the earth itself and also in the terrella, for magnetick proofs and demonstrations. For we consider the bodies themselves only, not the general appearances of the world. ”

“ Since the demonstration that the direct influence of the sun and the moon upon the earth, each being considered as magnets, is inappreciable, all attempts to construct a theory of terrestrial magnetism have been confined to causes within the earth's atmosphere. ”

“ But, you will ask, what is it on the Sun that correspends to the little magnet? and the answer has been given us in a beautiful manner quite recently, by Professor Hale, the great American astronomer, who has erected the Mount Wilson Observatory in California. His proof depends on experiments with polarized light which we cannot stop to explain just now, though I will show you some pretty experiments with it at the end of the lecture; but the gist of the matter is that a sunspot is an electric whirlpool, which is as good as a magnet. Many of you know already what an electro-magnet is ”

“ Hence the atmosphere is a highly magnetic medium, and this medium is changed in its magnetic relations by every change in its density and temperature, and must affect both the intensity and direction of the magnetic force emanating from the earth, and may account for the variations which we find in terrestrial magnetic power. ”

“ It is well to remark that the curves representing magnetic variations and auroras lag about six months behind those representing the sun-spot variations. The sun-spot area seems in some way to depend upon the position of the planets. Not only is the earth's magnetism thus, seemingly at least, influenced by the sun-spots, but also some of the phenomena of the weather. ”

“ As the refraction of the atmosphere in the polar circles, is at least ten times greater than in the tropics, a greater quantity of the magnetic rays will there be separated and combined than elsewhere; and of course arises excess of magnetism. ”

“ Magnets attract only when their poles are reversed, or when they are oppositely electrified, and suns and planets do the same. The sun as the great central magnet, or electric generator, has the power both to attract and to repel. The planets are kept in their orbits by both the sun's attraction and its repulsion. In the summer, when the earth gets four millions of miles nearer the sun, there is a repelling force from the sun that sends it off again, or it would continue to approach the sun until it would fall into his spacious bosom. ”

“ When spots are numerous on the Sun’s disc there is great disturbance of the solar surface, accompanied by fierce rushes of intensely heated gases. This solar activity is known to influence terrestrial magnetism by causing a marked oscillation of the magnetic needle, and giving rise to so-called ‘magnetic storms,’ accompanied by magnificent displays of auroræ, with variations in electrical earth-currents. ”

“ These methods imply that a complete magnetic survey of the earth's surface has been made, so that the values of or of or of both are known for every point of the earth's surface at a given epoch. What we actually know are the magnetic components at a certain number of stations. In the civilized parts of the earth these stations are comparatively numerous; in other places there are large tracts of the earth's surface about which we have no data. ”

“ A connection between the terrestrial magnetism and the auroral phenomena is further proved by the fact that, before the appearance of an aurora, the magnetic intensity of our globe considerably increases, but diminishes as soon as the first flashes show themselves. ”

“ Again, if a sunspot is regarded as a superficial source or sink of radial flow of strongly ionised material, with the familiar vortical features, its strong magnetic field would, on these lines, be a natural accompaniment and if it were an inflow at one level compensated by outflow at another level, the flatness and vertical restriction of its magnetic field would be intelligible. ”

“ It is inferred that magnetic disturbances, both solar and lunar, are produced mediately by the Earth, and that the Earth in periods of several years undergoes changes which fit it and unfit it for exercising a powerful mediate action.—The Earth-current records had been reduced, and the magnetic effect which the currents would produce had been computed. ”

“ Two or three times in 1882 the earth responded instantly with magnetic storms and brilliant auroral displays to the solar activity, but this year the great sun-spots and their accompanying phenomena have shown comparatively little power to affect terrestrial magnetism. ”

“ Whether this is the consequence of the currents of electricity, which Ampère supposes to circulate around the globe, from east to west, or the result of points of attraction in the earth itself, the phenomenon is equally wonderful. To whatever cause we may refer the visible effects, it appears certain that this earth is composed of particles in a magnetic state, the character varying with physical conditions, and that terrestrial magnetic force is the collective action of all the atoms of this planetary mass. ”