Popular Science Monthly/Volume 24/February 1884/The Aurora Borealis
|THE AURORA BOREALIS.|
HOW can we describe, how can an artist paint, the aurora borealis? We of temperate climates are not strangers to the phenomena; we know something of the arcs and radiating streaks of various-colored light which frequently adorn our northern skies; and we are occasionally permitted to witness exhibitions in which the whole heavens shine with their marvelous glow. Yet travelers from the far North say that we can have no conception of the wonderful splendor of the phenomena as witnessed within the Polar Circle, and that nothing but the actual sight can convey an adequate idea of it.
The aurora borealis was well known to the ancients. The Greeks, discovering graceful symbols in everything, thought it was the glory of the Olympian gods holding council in a sky illuminated for the occasion. The Romans, on the other hand, always looking for unlucky omens, were in dread of it. Pliny, following Aristotle and Seneca, speaks of celestial fires that tinged the sky with a blood-red, of beams of light, of openings yawning in the starry vault, of fantastic lights that changed night into day; and he took care not to omit the political events that accompanied such manifestations, without, however, affirming that the phenomena were the cause of the catastrophes that attended or followed them. At troubled seasons in antiquity and the middle ages, in times of war, famine, or epidemic, the only sentiment the aurora excited was that of fear, and the people thought they could see in the sky rivers of blood, armies clashing, and infantry and cavalry engaging in mysterious combats. Now, except among a few superstitious or uninformed persons, the phenomena are witnessed with simple curiosity by some, with indifference by others.
A thousand years after Gregory of Tours, who gave the meteor the name it now bears, Gassendi first examined it with a scientific eye, and definitively baptized it on the 12th of September, 1621. The terms "polar light" and "northern light," which have been proposed by various physicists, have never prevailed; the bishop and the philosopher have triumphed. From the beginning of the eighteenth century, observations became more numerous, and theories and scientific discussions began to appear. The subject even tempted the poets. To say nothing of the Abbé Delille, an Italian Jesuit, Father Noceti, sung the aurora in Latin verse. Frazier, in 1712, first witnessed a southern aurora.
It is affirmed that the aurora was not common in Scandinavia and Holland previous to about 1716, after which it began to appear more frequently. Whether this be so or not, the attention of several Swedish, Dutch, and French investigators was fixed upon it. Celsius, the designer of the centigrade thermometer, remarked the curious distractions to which compass-needles were occasionally subject, without visible cause; studying the perturbations more closely, he had no difficulty in assuring himself (1741) that they coincided with the appearance of the aurora borealis. Hjorter, another Swede, made the same observation at about the same time.
The question whether auroras are of cosmic origin, or whether they proceed from purely terrestrial influences, which still provokes discussion, has from the beginning divided the learned into two parties. Mairan maintained the extra-terrestrial character of the meteor, while the contrary opinion found a supporter in Musschenbroek, the inventor of the Leyden-jar.
Musschenbroek, still evidently under the influence of old middle-age prejudices, gave out the following hypothesis: Near both poles, and at a little distance beneath the surface of the globe, are immense reservoirs of phosphorescent matter. Whenever a fissure is formed reaching to them, the substances, readily volatile, escape and illuminate the atmosphere with their glow. The frequency of auroras in particular years was explained by supposing a subterranean cavern to have been opened. When the pocket was exhausted, the phenomenon would of course be at an end for some time. So, after the exhaustion of the provisions of phosphorescent stuff accumulated in a particular region, the meteors would necessarily cease to show themselves, not to appear again till after a long time, during which the matter would accumulate again. It was thought that years of dry weather were years of maxima of auroras, and it seemed natural to suppose that moisture would hinder exhalations. Extensive efforts were made, without success, by studying the properties of the recently discovered phosphorescent substances, to determine the nature of the stuff that thus shone in space. Previous to this, an explanation of the phenomenon had been suggested by supposing a fermentation of gross exhalations from the earth's surface which were driven toward the pole and there took fire.
Quite different from this was Mairan's theory; and the reading of his book, "Traité physique et historique de l'aurore boréale" ("A Physical and Historical Treatise on the Aurora Borealis"), which appeared in 1733, is still indispensable, after a hundred and fifty years, to any person wishing to study the meteor to-day. Rejecting the ideas outlined above, and another curious hypothesis, that the rays of the sun were reflected from the polar ice, and sent back to the observer from the concave surface of the upper strata of the atmosphere, he had recourse to the zodiacal light which had been observed by Cassini some fifty years before. While some explained this phenomenon by supposing a ring of light concentric with the sun, and surrounding it without touching it, others, and Mairan among the number, considered it a prolongation of the solar atmosphere, accumulated chiefly in the plane of the ecliptic or of the solar equator, and extending beyond the orbit of Venus. Emanations from the sun, or rather the corona that surrounds it, according to Mairan, strike our atmosphere and illuminate our globe. Then, must we suppose that the zodiacal light shines of itself? That is not necessary, says Mairan. A chemical combination, an essentially luminous precipitate, results from the mixture that takes place in the upper regions of the atmosphere. This supposition is hazardous, and Mairan seems to be a little too fast. It is, however, indisputable that then, as now, auroras were more frequent in March and September, or the months when the zodiacal light is brightest. It is also worthy of remark that Angström, in 1867, and Respighi, in 1872, found in the spectrum of the zodiacal light a green ray identical with a line of the same color characteristic of the aurora borealis.
Mairan found a redoubtable antagonist in the celebrated mathematician Euler, who did not admit the hypothesis of an immense solar atmosphere, and believed only in the existence of a ring. He invented, in explanation of the meteor, a somewhat obscure theory, according to which the subtile and rarefied portions of the air were driven away from the surface of the globe, and the particles, having become luminous (he does not say how), gave rise, at some distance from the earth, to the phenomena of the aurora.
A large library would hardly be sufficient to hold all the memoirs and notices that have been published during the past sixty years on the subject of the aurora borealis, to say nothing of the numerous treatises on physics, meteorology, and astronomy which have devoted one or more chapters to it. Some authors have limited themselves to the simple description of what they have perceived, or to a mere exposition of their theories, while others have done more. Alexander von Humboldt has drawn in his "Cosmos" an excellent outline of the ideas which science entertained on the subject in his time; and the "Popular Astronomy" of Arago contains valuable details, well classified and arranged, on the same question.
About 1850, M. de La Rive, a Genevese physicist, endeavored to found a definite theory of the aurora borealis, and with this view artificially reproduced the phenomenon with considerable success. A prime point, which is still far removed from being fixed, is the approximate height of the meteor above the ground. Sometimes two observers, in the neighborhood of a thousand miles apart, will affirm that they have seen the same aurora at the same time and under the same aspect; at other times, the phenomenon is visible only within a radius of a few leagues. Mairan, basing his calculations on data that are not without value, concluded an elevation of two or three hundred leagues; Bravais proposed one hundred and fifty kilometres as a mean value. Other authors have supposed that the highest flashes soar to an elevation of eight hundred kilometres.
M. de La Rive has made a table of all former data, and represents that the auroræ boreales, very low in reality, hardly pass beyond the zone of clouds. They have been perceived (by Parry) projected on the flanks of mountains. Contradictions of this view are also not wanting. In support of his opinion that the meteor is low in height, M. de La Rive cites the well-established cases in which sounds have been heard during the manifestations. Sometimes a sulphurous odor has been perceived. The crackling occasioned sometimes by slow electric discharges and the odor of electrified oxygen or ozone are quite analogous. Explorers and aëronauts have pretended, according to M. de La Rive, to have gone through the aurora or through the mist that gives rise to it.
Arago had conceived the electric nature of the meteor, and assumed to predict its appearance by consulting the compass. Other facts, proving a connection between auroras and magnetic phenomena, are abundant. Jessan, in 1878, sailing on the Venus, relates that during a fine aurora all the compasses of the vessel were disordered, and they went out of their way. Under similar circumstances, Matteucci observed the iron of the Tuscan telegraphic apparatus to be so strongly magnetized that the entire service between Florence and Pisa was interrupted. In the United States, when the like conditions are prevailing, the telegraphers work their instruments without the batteries.
The beautiful arcs of light which are observed in the polar regions have their culminating point on the magnetic meridian, as the vertical plane defined by the points of a horizontal magnetic needle is called. Bravais thought these arcs, or the circles of which they form part, were concentric with the magnetic axis of the globe, or with the straight line uniting the two magnetic poles and passing through the center of the earth. The arcs, then, do not coincide "with the geographical parallels, a fact which the earlier observers had already perceived. The magnetic pole is, moreover, not immovable, but its position may vary during a century several degrees in longitude or latitude.
The auroræ boreales certainly appear to be connected with a particular condition of the atmosphere, and M. de La Rive finds in this a confirmation of his theory. Nearly all the observers agree that cirrostratus clouds accompany or precede the phenomena, and are frequently seen within the dark segment. Hardly less invariable is the simultaneous presence in the air of hosts of fine, transparent, microscopic needles of ice, that favor the formation of lunar halos before the aurora itself breaks out. The essential points of M. de La Rive's theory are that the earth is charged with negative fluid, and the same is the case with the strata of air very near the soil. The upper regions of the atmosphere are, on the other hand, positively electrified. This double fact, the result of certain experiments, is not denied by any one. The two electricities of opposite polarity, accumulated near the tropics in enormous masses, are combined at the poles, where the air, less moist, is a better conductor. The polar discharges produce incessant calls of fluid, if we may use such an expression, and currents of electricity are constantly departing from the equator toward the poles, one kind traveling through the rarefied gases of the upper strata, and the other kind through the ground. It is from the phenomenon of recomposition, favored by the presence of infinitesimal vesicles of air, of imperceptible snow-crystals, and of little icy needles, that proceeds the meteor of which we are trying to present the history.
M. de La Rive satisfied himself of the sufficiency of his theory by an experiment. Tubes were inserted opposite to each other into the sides of a glass bottle. The air within the bottle was exhausted by means of one of the tubes, while in the other one was fixed a rod of iron projecting on the outside, and having its other end prolonged to the middle of the bottle. The iron was covered with an insulating material, except at the end, and over that was a copper ring, connected with an electrical machine. The copper was then charged with positive electricity, and the iron, having been put in communication with the soil, was negatively electrified by induction. The two electricities combined in the rarefied atmosphere of the bottle, forming a luminous sheaf, like that of the lights in the Geissler tubes; but, when the iron was magnetized, a corona or concentric aureole, whence radiated brilliant jets, was formed around its free end. As a little reflection will show, the iron represented the earth and the terrestrial magnet; the copper, the upper strata of air; and the free end of the magnetized rod, the polar regions.
The fact mentioned by Mairan, that auroras are most frequently seen during the equinoctial months, March and September, is easily explained on M. de La Rive's theory. March corresponds with a period of increasing heat in the tropical part of the northern hemisphere, while September coincides with the time when fogs are condensed from vapors near the pole. In the one case, an excess of electricity is developed; in the other, a more ready combination of the two fluids. Perhaps the supposed eleven-years period, corresponding with the sun-spot period, may be explained in a similar way. There may also be secular variations in the prevalence of the phenomenon, but too little time has passed since careful observations have been made for their law to be as yet apprehended.
In the last months of 1878, M. Nordenskjöld, who was wintering in Berhing Strait, remarked on clear nights, when the moonlight was not too strong, the presence of a feebly luminous arc, with its crest toward the north-northeast. Regular in form and curvature, this arc rested on a segment of a circle which was itself limited by the horizon, and covered about 90°, or a quarter of the horizon. Its lower limit was quite clearly marked on the dark segment, probably by contrast, but its outer outline was less distinct, and it was hard to measure its thickness exactly; but that was estimated at about five degrees. The light of the arc was calm and uniform, without any appearance of rays, but dull enough, as we have said, and displayed nothing comparable to the draperies, the brilliant flashes, and the streaks of the Scandinavian auroras. M. Nordenskjöld observed it from day to day, taking notice of all the special features he could remark, and came to the following conclusions: Above the surface of the earth, at a distance of about four hundred kilometres, is situated a permanent, or nearly permanent, luminous corona, which encircles the entire globe without its direction coinciding with that of the parallels, for its center does not correspond with the north pole, but with the magnetic pole.
So our globe has, by this theory, a ring like Saturn's, but with some differences. The ring of the latter planet is around its equator. Our ring, incomparably smaller, covers only a narrow zone of the polar regions, the center of which is at a considerable distance from the pole. The inhabitants of Saturn's equator—if there are any—look out upon a ribbon very wide in the vertical but very narrow in the horizontal direction. On the other hand, an observer in the high latitudes of Asia or America stands in the presence of a corona of little thickness, but relatively extensive; that is, the development of our ring is nearly parallel to the part of the terrestrial surface dominated by it, and which it would overshadow if it were opaque.
To this theory the objection may be offered, that no one before M. Nordenskjöld has remarked the meteor in question, while many should have done so if it is permanent. An observer standing near the auroral pole should perceive a luminous circle completely enveloping the horizon. M. Nordenskjöld replies to this by saying that the luminous arc is only a residuum of more brilliant and more complex phenomena; we can hardly hope to see it except in years when auroras are weak, or years of minima, of which the year 1878-'79 was one. Most commonly the accessory masks the principal, much in the same way that we can not see the foundations of a house while the building is standing. The light of the ring is so weak that not only the day and the twilight, but simple moonlight makes it invisible. If the sky is charged with frost, it will all disappear, and even the presence of too much vapor in the air extinguishes it. The observer must, then, be favored with dry and cold weather. If the temperature is above the freezing-point, it is useless to look for the corona. The coasts of Norway, moist with the breezes from the Gulf Stream, are badly situated to give views of it. Nearly all other regions where it could be perceived are dismal solitudes. In the second place, a spectator situated near the auroral poles would see nothing, for the horizon would hide the meteor from him in the same way that a Saturnian, who never left the high polar regions of his planet, would not be aware of the existence of his ring. Our observer, leaving the auroral pole, and going toward the magnetic south, would finally distinguish in that direction an arc gradually rising above the horizon. An entire circle of considerable width is dominated—that is the word—by the corona, which is then near the zenith; but, although the meteor may be nearer the ground at that point than anywhere else, it is not visible there, for it is too thin to be seen, looking at it vertically. Outside of this latter zone, another zone, concentric with it, enjoys the sight of the arc, now situated obliquely in the direction of the magnetic north. Further on, the arc, grazing the horizon, ceases to be visible; some time before reaching this point, in fact, it is hidden by the mists that gather in the horizon, as well as by the density of the atmosphere which the visual rays have to traverse. M. Nordenskjöld would not have been able to see it if it had been only half as luminous.
The meteor is relatively stationary, but is not rigorously motionless. Besides the slow variations of its radius and its thickness, besides the oscillations which displace its center movements, the laws of which are worth studying, the luminous arc rises, falls, and fades away for intervals of some hours. Its light, generally uniform, is heightened by "knots of light" that play from one end to the other. Sometimes a second arc is formed parallel to the first; according to M. Nordenskjöld, this is nearly always concentric with the usual arc and situated in the same plane with it, but farther from the surface. Sometimes, also, the two arcs amalgamate, and a vertically flattened aurora results. Not rarely, supplementary arcs intervene, and frequently luminous rays play between the two arcs and into the undefined exterior space. If, now, we imagine the phenomena growing more complicated and becoming irregular, with the arcs rising above the horizon and the rays multiplying, shooting through the curves in such a way as to illuminate the vacant space, and extending themselves out toward the magnetic south in somewhat oblique directions, we have the common aurora borealis passably explained. Within the projection of the corona, toward the magnetic pole, is a zone where we may observe the auroras in a southerly direction, and, still nearer to that pole, the meteor only rarely illuminates the horizon. A few travelers, Dr. Hayes, for example, noticed this fact some time ago. The zone of no auroras embraces a circle having a radius of about eight degrees.
The labors of M. Lenström, in Lapland, are of particular interest, because they constitute a direct and definite proof of the electrical nature of the aurora borealis. They go further than those of M. de La Rive, for the Swedish observer, instead of operating in his laboratory, has succeeded in reproducing the meteor itself in the open air, and has compelled it to manifest itself, as Franklin forced the lightning to come down from the sky, so that he could examine it scientifically. We must not forget, furthermore, that it is a very meritorious thing to work in a cold of twenty degrees below zero, with a strong wind blowing and the frost all the time clogging the apparatus, having to be constantly on the watch, and enjoying no better shelter than a charcoal-burner's hut.
Not satisfied with provoking artificial auroras, the Finnish expedition, of which M. Lenström was a part, has collected a number of important data relative to the free manifestation of the phenomenon. The observations were made at Sodankylä (lat. 67° N., long. 27° E.), and Kultala (lat. 78° 30' N., long. 27° E.), Lapland, in November and December, 1882. In the former place "the polar aurora appeared frequently of a very great intensity, but did not exhibit much variation. It generally began with a faint arc in the north, which shortly developed into an arc with rays and sometimes into draperies extending from the east to the west, most frequently a little toward the north. But little change of color took place; nearly always a pale-yellow tint, lightly washed with green, was shown. Although the meteor was not visible continuously, there was often observed in the spectroscope, and even quite high above the horizon, the characteristic band of the auroras without the eye perceiving any trace of their light. Since this fact was remarked even when there was no snow, it could not be attributed to reflected auroral flashes. Moreover, the observers not rarely saw during the nights a light yellowish, diffuse, and phosphorescent light that illuminated the horizon and paled the stars. The effect produced was compared to that of the moon half veiled by clouds. M. Lenström and his associates attempted, on the 8th of December, 1882, to measure the height of an auroral arc above the surface of the earth. They divided themselves into two groups, and took with a theodolite the angular distance from the crest of the arc to the horizon. The two stations were four and a half kilometres apart on the same magnetic meridian, and correspondence was had during the observations by a telegraphic wire previously arranged for the purpose. They endeavored to look in concert at the same point of the meteor, but, after reiterated essays, they recognized that any particular ray visible to one party could not be seen by the other. The results of the views were irreconcilable, for the angle obtained was greater for the southern post than for the northern one, although the latter post was, a priori, nearer to the meteor. M. Lenström concluded from this, as M. de La Rive had done, that every observer sees his own aurora the same as every one sees his own rainbow, and that the phenomenon is produced at the height of only a few thousand metres. He also calls attention to the results obtained in Greenland by the engineer Fritze, which lead, in certain cases at least, to numbers twenty times as small. During the Swedish Polar Expedition of 1868, faint flashes or phosphorescent lights were remarked around the summits of the mountains. This fact, with which M. Lenström did not become acquainted till 1871, related as it was to some of the descriptions given by other travelers, decided him to try to provoke or facilitate the appearance of the meteor by artificial means. The first attempts date from 1871, and, like those that followed them, were made in Lapland. The enterprise being successful from the first, the experiments were resumed during the Finnish Polar Expedition of 1882, and were renewed twice on two different peaks, called respectively Oratunturi and Pietarintunturi. Oratunturi, rising more than five hundred metres above the level of the sea, is situated in latitude 67° 21', near the village of Sodankylä. Near the topmost height of the mountain was placed a long copper wire, so bent upon itself as to form a series of squares within squares, having a total surface of nine hundred square metres, supported by insulated posts. Tin points or nibs bristled out from this connecting net at distances of half a metre apart, and the whole was connected by an insulated wire running along on stakes with a galvanometer fixed in a cabin at the foot of the peak. The galvanometer was connected with the earth by the other tremity of its own circuit. Nearly every night after the installation of the apparatus, a yellow-white light illuminated the points without anything like it appearing on the heights in the neighborhood, while the needle of the galvanometer by its motions betrayed the passage of an electric current. The light was analyzed in the spectroscope, and gave the greenish-yellow ray that characterizes the aurora borealis. The
intensity of the glow and the deviations of the needle, moreover, varied continually. In the mean time the hoar-frost which was deposited on the wires quickly destroyed the insulation, and rendered an experiment of any duration almost impossible. The numbness of the fingers of the operators, induced by the cold, added to the difficulties of the study.
The apparatus afterward set up on Pietarintunturi, in more than 78° of latitude, was disposed in an almost identical manner, except that the surface furnished with points was a half less; but, M. Lenström remarks, the proximity to the "maximum zone" of auroras compensated for this inferiority. On the 29th of December an "auroral ray" made its appearance above the net, which it dominated vertically from a height of one hundred and twenty metres.
The difficulties of the question of the exact origin and nature of the auroral phenomena have not been solved yet; but we have good reason to believe that a long approach has been made in the recent experiments toward a solution, and grounds to believe that science will soon remove them all; and we shall no longer be able to repeat what Haüy, less than a hundred years ago, said on the same subject, "It is not always what has been known longest that is best."—Translated for the Popular Science Monthly from the Revue des Deux Mondes.
- Mairan observes that, the centrifugal force being less toward the poles than at the equator, the parts of the globe at the tropics will repel the foreign matter, and it will accumulate toward the high latitudes. Hence there will be few auroras except in the frigid and temperate zones; and this is the case.
- In this Euler made use of Newton's corpuscular theory of light, though he was an adversary of it.
- Nevertheless, if the observer is within the circle formed by the aurora, its action on the needle is almost nothing. This fact has been noticed more than once.
- Professor Lenström's apparatus is represented in the figure. The wire begins at 0, and connection with the galvanometer is made from the inner end. The letter i indicates an insulator.