Popular Science Monthly/Volume 36/April 1890/Northern Lights
THE inhabitants of northern Europe, who passed their days in the midst of vast forests, and whose fancy fashioned the forms of heroes and of gods from the mists that hung over their vales, naturally associated with the gods they worshiped the phenomena of northern lights, which to them were revealed in all their splendor. Thus, the Edda gives descriptions of naming steeds speeding to Walhalla, and of valkyries dashing on through seething flames. Nations that as yet rest close to Nature's breast do not seek explanations of such phenomena; while those that have risen to a higher plane of culture are in possession of simple descriptions of these occurrences, and also of crude attempts at investigating Nature's wonders. Thus, in the old writings of the Chinese, whose realm was a flourishing one two thousand years before our time, there may be found many accounts of the occurrence of northern lights. They observed red vapors arise in the northern heavens, which spread evenly to both sides; sometimes the fiery sheen was encircled by a large white bow, and flaming rays pierced the vapors. Such descriptions can only refer to northern lights.
In the Greek and Latin classics we find more detailed descriptions of similar phenomena. Aristotle, Cicero, Seneca, Pliny, Lucan, Plutarch, Tacitus, and others describe the appearance of auroras more or less completely. Some authors, instead of giving a simple narration of events, seek for an explanation of what they observed. Of course, these attempts in the main are very naïve and without any scientific value.
Aristotle speaks of red beams in the heavens, of torches and of billows of fire. Seneca compares the phenomena to flashes of lightning, and writes about the blazing of the heavens. According to him, "the gleaming flashes may be caused by violent winds, or by the heat of the upper regions of the air; for, when the fiery phenomenon spreads far, it sometimes extends to the lower region, if it be inflammable."
Pliny writes: "Fiery beams occur likewise; such a one was seen when the Lacedæmonians, vanquished at sea, lost their sway over Greece. Sometimes the heavens cleave; this is spoken of as a 'chasma.' But naught is more terrible for mortals than when a blood-red conflagration starts in the heavens, and from there falls to the earth. This happened in the third year of the one hundred and seventh Olympiad, when King Philip warred in Greece. I, however, believe that these phenomena, as all others, occur at times regulated by Nature, and are not, as most people suppose, to be ascribed to a variety of causes which their fancies invent. They have, however, been premonitors of great misfortune. As they occur so very rarely, the law which they obey remains hidden, and may not be traced" Furthermore, "During the reign of the consuls Caius Cæcilius and Cneius Papirius, and also at other times, light was seen in the heavens, so that night became as day." The words of Lucan, "Fire storming from the north," remind us of the sagas of northern tribes.
The middle ages could not readily free themselves from the influence of the mysterious. Wondrous phenomena, the true nature of which was not grasped and understood, were veiled in mystery, and ascribed to the workings of demons. This is proved by many records and traditions. Does not Shakespeare possibly refer to northern lights in "King Henry VI," Part III, act ii, scene 1?—
Rich. See, how the morning opes her golden gates,
And takes her farewell of the glorious sun!
How well resembles it the prime of youth,
Trimm'd like a younker, prancing to his love!
Edw.Edw. Dazzle mine eyes, or do I see three suns?
Edw.Rich. Three glorious suns, each one a perfect sun;
Not separated with the racking clouds,
But sever'd in a pale clear-shining sky.
See, see! they join, embrace, and seem to kiss,
As if they vow'd some league inviolable:
Now are they but one lamp, one light, one sun.
In this the heaven figures some event.
Many investigators consider the observations of Gassendi, which relate chiefly to the phenomena of 1621, as the starting point toward a more correct conception of the nature of northern lights. The first move toward a truly scientific investigation into the matter was made by Halley, who in 1716 suggested that auroras were a magnetic exudation from the northern pole of the globe. His contemporaries did not share Halley's opinion. Wolf, in Halle, maintained that the lights consisted of inflammable sulphurous fumes. Descartes and Triewald saw in them only a reflection of the snow and ice at the north pole. Mairan (1733) considered them formed by a blending of zodiacal light with the earth's atmosphere. The famous mathematician Euler adhered to a sort of nebular hypothesis and declared the aurora to be a phenomenon similar to that presented by the tails of comets. Halley had arrived at his view through the observation that the center line of the light-arc deviated to the west of the meridian to about the same extent as the north pole of a magnetic needle. This important discovery was followed by one made by Mairan, that the crown of the northern lights lies in the (prolonged) direction of the dipping-needle; and soon after this Hjoter, in Upsala, demonstrated the influence of the aurora on a magnetic needle placed horizontally (1741).
It was by these discoveries that the relation between northern lights and magnetism was established. Winkler (1746) and Van Marum (1777) compared the former to the electric glow which can be produced in rarefied air. The veil of the mystery had been raised, but only to disclose a new query, for the demonstration of the cause of these relations was a problem the solution of which was reserved for modern science. How far the efforts in this direction have been crowned by success we are now to consider.
Auroras are most frequently seen in the cold and in the northern temperate zone, rarely in the southern temperate zone, and hardly ever in the tropics. The places where they most frequently appear lie between the sixtieth and seventieth degrees of north latitude. In the form of an oval, they include the geographical as well as the magnetic north pole, which is to be found on the peninsula Boothia Felix, Iceland, the Kara Sea, northern Siberia, Bering Strait, Hudson Bay, Labrador, and Greenland. Northern lights have been seen as far down as the twenty-fifth degree of north latitude. In full splendor, however, they may be seen only in the northern polar regions to the seventy-fifth degree of latitude. Here Nature is displayed in all her grandeur. When the sun has set, and the gray veil of twilight is cast over the earth, the northern horizon grows darker and darker. Soon there may be distinguished a segment of the sky more somber than its background; this is hemmed in by white concentric arcs of light. Colored rays shoot forth therefrom in all directions. They interlace and intertwine until they seem to be woven into one fluttering band of color. And anon it changes to a sea of fire! The rays leap upward far above the zenith, form there a flaming crown, and then sink back, to begin anew the wondrous play. The most beautiful descriptions and trustworthy observations we owe to Nordenskjöld and Ekama. All explanations which have been attempted as to the origin and the cause of northern lights are based on analysis by the spectroscope, on the determination of the elevation, and on observations of the peculiar behavior of the magnetic needle.
If northern lights are observed through a spectroscope, a characteristic yellowish-green line will always be seen, no matter how faint the light may be. If the phenomena are well pronounced, several red lines appear in the spectrum. What inferences may be drawn from these observations?
If a very powerful electric current is sent through what is called a Geissler tube filled with dried and rarefied air, sparks will flash from one platinum wire to the other, and if these are examined by the spectroscope they show the characteristic yellowish-green line. In a tube filled with nitrogen these lines are especially well pronounced. As nitrogen is the predominant constituent of the air, we infer the northern lights to be a phenomenon similar to the glowing of the electric spark in dry and rarefied air, in which oxygen and nitrogen are the sole conductors of electricity. If white sunlight is caused to shine on a solution of chlorophyl (the green coloring matter of leaves), it shows a blood-red color. This phenomenon, when the reflected light appears of a different color from that which is transmitted, is termed fluorescence. Electric light possesses to a marked degree the property of calling forth fluorescence, as can easily be demonstrated by various experiments. It may hence be assumed that the red lines in the spectrum of northern lights are due to the fluorescence of oxygen in the upper layers of air, caused by powerful electric discharges.
The determination of the elevation of northern lights serves to confirm the foregoing conclusions, for it is a well-known fact that in the higher regions of the atmosphere the air is dry and rarefied. The height of the auroral crown has been found to be five hundred kilometres, the lower boundary being about seventy kilometres.
Of greater importance for ascertaining the true nature of northern lights has been the demonstration of the relation these phenomena hold to terrestrial magnetism, a mysterious power of which our globe is the conductor. Powerful magnetic currents appear suddenly and disappear as quickly, and we are not able to name the cause of their existence.
Like a vast ocean the magnetic force is constantly surging through our globe, here appearing, there receding, but forever seeking to attain its equilibrium. The instrument by which these occurrences are observed is the most simple conceivable—the magnetic needle. It is known that the direction and the inclination of the needle point out the magnetic location of a place. From its movement the degree of magnetic attraction of any point may be accurately determined. If sudden deviations and fluctuations of the needle occur, this evidently is an indication that the magnetic condition of the earth has been disturbed. If such variations always occur simultaneously with the appearance of northern lights, this is to be regarded as proof that an intimate relation exists between these phenomena and terrestrial magnetism. Strong northern lights, in fact, exert so great an influence on the magnetic condition of the earth, that the appearance of the phenomenon in some northern latitude may be inferred from the sudden oscillations of the needle in places where the lights are not visible. But the most striking connection between terrestrial magnetism and northern lights is shown by the position of the light-crown in the heavens. The center of this is always to be found at that point where the dipping-needle, if prolonged, would meet the aurora. The rays emanating from the arc have the direction of the earth's magnetic power; they are therefore parallel, and only apparently converge to a point. The crown of light has in truth no fixed place in the heavens, but like the rainbow its position depends on the point from which it is observed, and thus moves with the observer. But the dipping-needle in every place points to the center of the crown.
It may hence be asserted that the northern lights hold an intimate relation to the changes of terrestrial magnetism as indicated by the oscillation of the needle, and that both phenomena must have one and the same cause. The explanation of this cause has been sought in various ways, but an interpretation which would be universally accepted has thus far not been advanced. One of the first attempts to solve the problem was made by the physicist De la Rive, of Geneva, in 1862. It was based on an experiment devised by him, in which he attempted an imitation of these phenomena on a small scale. He held that vapors charged with positive electricity rise into the higher regions of the atmosphere, while the earth remains charged with the negative fluid. When the vapors are driven by the trade-winds to the poles, as soon as the tension is sufficient to overcome the non-conducting property of the air, which like an insulator lies between the earth and these vapors, the positive and the negative electricity come together. This process is accompanied by the appearance of light. The earth and the upper layer of the atmosphere must hence be regarded as an electrical condenser, with the lower layers of air as the separating medium. An actual proof that the northern lights are caused by electric currents in the atmosphere was attempted by Lemström in 1883. He covered the plateaus of two mountains in northern Finland with a network of copper wires raised several metres above the ground and provided it with hundreds of metal points. The whole was insulated and connected with a zinc plate buried in damp ground in the plain below. A continuous electric current from the air to the ground was noticeable, and a light which appeared hovering over the metal points showed, when examined by the spectroscope, the characteristic line of the auroral spectrum.
The theories, however, according to which the northern lights are a flowing together of terrestrial and atmospheric electricity of opposite kinds, leave unanswered the question as to the origin of these electric fluids. As no adequate cause could be found on the globe for such a tremendous evolution of electricity, attention was directed to the sun as the source of it all. Why should not Helios, the giver of all light and life of our world, be as well the creator of that inexhaustible force of nature that is revealed in the splendors of the northern lights?
As the endless supply of light and heat which is radiated into space by the sun, is accounted for by the contraction of that body, this may also be assigned as the cause of the stupendous generation of electricity. According to the theory of Kant and Laplace, the sun and other heavenly bodies are assumed to have been formed by the condensation of vapors which originally filled all space. This condensation is still going on in the sun in consequence of the enormous radiation of heat into space, and with it the consequent contraction.
Possibly also there might be suggested as a cause the cooling process which the sun is undergoing. It may be assumed, too, that vast amounts of electricity are hurled into space with the ignited masses of gas, whose eruption from the sun may be constantly observed. But it is more probable that the sun acts upon the earth by induction. Try the following experiment: Two insulated spheres are placed near one another, but without being in contact. On one of these spheres a bar of metal is placed, to which there is fastened a screen made of some good conducting material. If one of the spheres is charged with a certain kind of electricity, say, for instance, negative electricity, the opposite kind—in this case positive electricity—will, by induction, be generated on the other sphere. A corresponding amount of negative electricity will in the mean time be discharged on to the screen. An action similar to this may be assumed to be going on between the sun and the earth.
The sun's electricity, which may be assumed to be negative on account of the preponderance of metal in the composition of that body, generates positive electricity on the earth by induction, while the negative electricity passes over into the atmosphere surrounding the earth. The constitution of the air at different times and at different places favors this process more or less. The conducting metallic rod of our electrical experiments may be considered here as being replaced by mountain-peaks projecting high into the air.
If the equatorial currents, already mentioned by De la Rive, are borne in mind, it is evident that the air at the poles must be highly electrified, and that an exchange must then and there take place between the negative electricity and the positive (induced) electricity. This exchange gives rise to phenomena of light identical with those observed as northern lights at the north pole and as southern lights at the south pole.
This theory is strengthened by the observation that northern lights are closely connected with the appearance of sun-spots and protuberances on the sun. Already in former times a certain periodicity of northern lights was noticed. Besides the annual period in which they appear most frequently at the times of the equinox, and least frequently at the times of the solstices, a period of eleven years has been observed, corresponding closely to that of the sun-spots, the maximum of which coincides with the maximum of northern lights.
The appearance of sun-spots and protuberances, the connection between which was pointed out by Tacchini in 1885, may be regarded as signs of changes occurring on the sun, probably involving increased combustion.
This increased activity must influence the induced terrestrial electricity, and in consequence also the phenomena of northern lights. The variations and declinations of the magnetic needle, moreover, give indubitable proof of the connection between the periods of sun-spots and the electrical condition of the earth.
It should not seem strange that the terms electric and magnetic condition have been here used as synonymous. Since Oersted 's discovery of the influence of the electric current on the magnetic needle, and Ampère's theory of magnetism, electricity and magnetism are regarded as merely two different forms of one and the same force of nature.—Translated for the Popular Science Monthly from Ueber Land und Meer.