Magnetic equator

“ No doubt the earth is a magnet on a vast scale, but it differs 343from all others in having four poles of maximum magnetic force of different intensities, the two in the northern hemisphere having a secular motion in a contrary direction from the two in the southern. They are not even symmetrically placed; hence the magnetic intensity varies so much in the different points on the earth’s surface, that the dynamic equator, or line passing through all the points of least intensity, is a very irregular curve surrounding the globe, but by no means coinciding with the terrestrial equator. ”

“ Downwind, away from the Sun, the magnetosphere extends much farther; some scientists postulate that a magnetotail may reach as far as the orbit of Saturn.
Charged particles in the magnetosphere are subject to powerful forces. Tightly embedded in Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particles are caught in the spinning field and accelerated to high speeds. The result is a co-rotating plasma in the magnetic equator of Jupiter, extending outward to at least 20 RJ. Beyond this distance, the flow breaks up and the magnetosphere is more unstable. ”

“ Thus in temperate latitudes where V is considerably in excess of H, whilst diurnal changes in V are usually less than those in H, it is the latter which chiefly dominate the diurnal changes in inclination. When the H influence prevails, I has its highest values at hours when H is least. This explains why the dip is above its mean value near midday at stations in Table XI. from Pavlovsk to Parc St Maur. Near the magnetic equator the vertical force has the greater influence. ”

“ Comparing Biot’s examination of the dip observations previously made by Humboldt, Krafft simplified the former’s conclusions, showing that if we measure the latitude from the magnetic equator, the tangent of the dip is double the tangent of such latitude, or, as he expresses it: “If we suppose a circle circumscribed about the earth, having the two extremities of the magnetic axis for its poles, and if we consider this circle as a magnetic equator, the tangent of the dip of the needle, in any magnetic latitude, will be equal to double the tangent of this latitude.” ”

“ When the ion is moving with a velocity small compared with the velocity of light, the lines of electric force are nearly radial, but as the speed of light is approached, they tend to leave the axis of motion and to bend towards the equator. When the speed of the body is very close to that of light, the magnetic and electric field is concentrated to a large extent in the equatorial plane.
The presence of a magnetic field around the moving body implies that magnetic energy is stored up in the medium surrounding it. The amount of this energy can be calculated very simply for slow speeds. ”

“ In applying the theory to the magnetism of the earth, we must bear in mind that the earth is probably magnetic by induction, and not in virtue of its own specific action. The rotation of the surrounding ether, and the consequent production of a radial stream, calls the ether into motion within the earth’s interior, as well as on the surface; but it does not follow that the ether shall also enter the earth at its poles and escape at its equator, for the obliquity of the vortex would interfere with this result. ”

“ The configuration of our globe, the distribution of temperature in its interior, the tides and currents of the ocean, the general course of winds, and the affections of climate—all these are appreciably constant. But magnetism, that subtle, undefinable fluid, is perpetually undergoing a change, and of so rapid a nature, that it becomes necessary to assume epochs, which ought not to be more than ten years apart, to which every observation should be reduced. The extreme importance of knowing the exact amount of magnetic variation can scarcely be overrated for maritime purposes ”

“ Now, as a rule, magnetism disappears at red heat, a temperature which would be reached, on the basis of the usual geothermal gradient, [8] at a depth of about 15 to 20 km. The strong magnetism of the floor of the ocean must therefore occur in the uppermost layers, which appears to agree quite well with our assumption that there the weaker magnetic substances are quite absent. ”

“ Since the earth's surface is diversified by elevations of land and depths of seas, great continental lands, oceans and seas differing in every way while the power that produces all magnetic movements comes from the constant magnetic earth-substance which is strongest in the most massive continent and not where the surface is water or fluid or unsettled, it follows that toward a massive body of land or continent rising to some height in any meridian, there is a measurable magnetic leaning from the true pole toward the east or the west. ”

“ And these may be expected in like manner to produce non-periodic occasional disturbances of the magnetism of the earth. Now, it is well known that such disturbances do occur; and, further, that they are most frequent in those years when cyclones are most frequent; that is to say, in years of maximum sun-spots. In one word, it appears to be a tenable hypothesis to attribute at least the most prominent magnetic changes to atmospheric motions taking place in the upper regions of the atmosphere where each moving stratum of air becomes a conductor moving across lines of magnetic force ”

“ Terrestrial Magnetism 1. Field Work. (a) Dip determinations were made at Macquarie Island, on the eastern and southern journeys from the Main Base (Adelie Land) and on a short journey from the Western Base (Queen Mary Land) . (b) Declination by theodolite observations was determined at Macquarie Island and at intervals on all sledging journeys in the Antarctic. (c) Rough observations of magnetic variation were made daily on the 'Aurora' during her five cruises. 2. Station Work. (a) Regular magnetograph records were kept at the Main Base (Adelie Land) for a period of eighteen months. ”

“ At midsummer the principal maximum is found—as at Kew or Greenwich—about 10 or 11 a.m., the principal minimum about 4 p.m.
§ 18. Even at tropical stations a considerable seasonal change is usually seen in the amplitude of the diurnal inequality in at least one of the magnetic elements. At stations in Europe, and generally in temperate latitudes, the amplitude varies notably in all the elements. Table XIII. gives particulars of the inequality range of declination derived from hourly readings at selected stations, arranged in order of latitude from north to south. ”

“ This theory is not yet definitely proved, but it furnishes a sufficient theoretical basis for future research. The earth itself is a gigantic magnet, a fact which makes the compass possible, and it is well known that the earth's magnetism is affected by those great outbreaks on the sun called sun-spots. Now it has been recently shown that a sun-spot is a vast whirlpool of electrons and that it exerts a strong magnetic action. ”

“ Apart from the surprising velocity of transmission of earthquake waves, the strongest argument for an iron core to the lithosphere is found in the magnetic property of the earth as a whole. The only magnetic elements known to us are those of the heavy meteorites—iron, nickel, and cobalt,—and the earth is, as we know, a great magnet whose northern pole in British America and whose southern pole in Antarctica have at last been visited by Amundsen and David, respectively. ”

“ 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. ”

“ The men at these stations could do good work for meteorology, by observing the periodic recurrence of Arctic phenomena, as well as any deviation from the ordinary rule, and would thus be enabled to discover the reasons for the alternation of storm and calm at the equator. The connection between terrestrial magnetism and atmospheric electricity, cable-currents, and the aurora borealis, can only be investigated in such high latitudes; while the laws of terrestrial magnetism itself will never be thoroughly appreciated unless the variations of magnetism in the far north are studied. ”