Aurora Borealis (more correctly Aurora Polaris, since the phenomenon is not confined to northern latitudes), called also Northern Streamers and Northern Lights, a luminous appearance, associated with energetic disturbances of the earth's magnetism and electrical condition. It is seldom seen save in high latitudes, though occasionally the tropics are visited by auroral displays. In polar regions auroras are very common, and usually far more brilliant than in the temperate zones. Humboldt gives the following description of the appearances presented when the auroral phenomena are fully developed, although it must be understood that there is considerable variety in these displays: "An aurora borealis is always preceded by the formation of a sort of nebular veil which slowly ascends to a height of four, six, eight, or even to ten degrees. It is toward the magnetic meridian of the place that the sky, at first pure, commences to become brownish. Through this obscure segment, the color of which passes from brown to violet, the stars are seen as through a thick fog. A wider arc, but one of brilliant light, at first white, then yellow, bounds the dark segment.
Sometimes the luminous arc appears agitated for entire hours by a sort of effervescence and by a continual change of form, before the rising of the rays and columns of light, which ascend as far as the zenith. The more intense is the emission of the polar light, the more vivid are its colors, which from violet and bluish white pass through all the intermediate shades to green and purple red. Sometimes the columns of light appear to come out of the brilliant arc mingled with blackish rays similar to a thick smoke. Sometimes they rise simultaneously in different parts of the horizon; they unite themselves into a sea of flames, the magnificence of which no painting could express, and at each instant rapid undulations cause their form and brilliancy to vary. Motion appears to increase the visibility of the phenomenon. Around the point in the heavens which corresponds to the direction of the dipping needle produced, the rays appear to assemble together and form a boreal corona. It is rare that the appearance is so complete and is prolonged to the formation of the corona; but when the latter appears, it always announces the end of the phenomenon. The rays then become more rare, shorter, and less vividly colored.
Shortly nothing more is seen on the celestial vault than wide, motionless nebulous spots, pale or of an ashen color; these disappear while the traces of the dark segment whence the phenomenon originated remain still on the horizon." Although auroras are more commonly seen in high latitudes than near the tropics, it is not toward the true poles of the earth that the increase takes place, nor does the increase continue after certain high latitudes have been reached. Thus the frequency of auroras is different at different stations in the same latitude; and in passing poleward from places in a given latitude, the region of maximum frequency is reached more quickly in some longitudes than in others. Thus an inhabitant of St. Petersburg would have to travel to lat. 71° N. before reaching the place of greatest auroral activity; while an inhabitant of Washington need travel northward only to lat. 56° to reach the region where auroral displays are most frequent. The zone on the earth's northern hemisphere where auroras occur most commonly and attain their greatest splendor, may be represented by constructing a ring of card or paper, of such dimensions as to agree with the 60th parallel of north latitude, and then pushing the ring southward on the side of America and northward on the side of Asia, until it passes through the most southerly part of Hudson bay and the most northerly part of Siberia. The position of the corresponding zone in the southern hemisphere has not yet been determined; but it is believed that the southern zone of maximum auroral frequency is nearly antipodal to the northern zone.
From what we know of the connection between the occurrence of auroras and disturbances of the earth's magnetism, we have every reason to believe that as the magnetic poles of the earth are slowly shifting, so the zone of maximum auroral frequency must also change in position. It cannot be doubted, for example, that in the 17th century, when the northern magnetic pole lay between England and the north pole, terrestrial conditions were more favorable for the occurrence of auroras in England than they now are, or than they then were in corresponding latitudes in North America. At present, on the contrary, the northern magnetic pole lies between the north pole and the northwestern extremity of the American continent; hence auroras are more frequent and more brilliant in North America than in corresponding latitudes in Europe. - To the description given by Humboldt we should add that sometimes in high latitudes, instead of extending from the horizon, the auroral arch appears in the form of a complete oval. Hansteen relates that at Christiania he twice saw the auroral arch in this form. Sometimes more than one arch has been seen.
Thus the observers who were sent by the French government to winter at Bos-sekop in Finland, saw on one occasion no fewer than nine arches, separated by dark spaces, "and resembling in their arrangement magnificent curtains of light, hung behind and below each other, their brilliant folds stretching completely across the sky." The position of the luminous region is not known. Arago was of opinion that each observer sees his own aurora, somewhat as each observer of a rainbow sees the luminous arc differently placed. Sir John llerschel says "no one can doubt that the light of the aurora originates nowhere but in the place where it is seen." But it has been considered that the most favorable conditions for the determination of the height of auroral gleams are presented when the auroral corona is formed. Now this corona always surrounds the point toward which the magnetic dipping needle points. Yet the magnetic dipping needles at different stations are not directed toward one and the same point; so that whatever the auroral corona may be, it does not seem to hold a definite place, in such sort that its distance can be determined by simultaneous observations; for it is the essential principle of the method of simultaneous observations that the lines of sight should be directed to one and the same point.
Nor is it easy, on Herschel's theory, to interpret the fact that the auroral corona has been seen at stations distant more than 1,000 miles from each other, and always around the part of the heavens pointed to by the magnetic dipping needle. For a point immediately overhead at one station, and 100 miles from the earth's surface, would be below the horizon of a station 1,000 miles distant. We seem forced to adopt the conclusion that though there is no analogy whatever between the aurora and the rainbow, yet Arago was right when he asserted his belief that as each observer sees his own rainbow, so each observer sees a different aurora. We should thus be led to consider whether the nature of the luminous emanations - the direction, for instance, of the luminous flashes composing them - may not explain the formation of the auroral corona. In this case the position of the observer would affect the appearance of the phenomenon. - If we assume that reliance can be placed on the observations by means of which the height of the auroral arch has been estimated, we must assign a considerable elevation to many of these lights.
On Oct. 17, 1819, an aurora was observed simultaneously at Gosport, Keswick, and Newtown Stewart, in Great Britain; and from the calculations made by Dalton the meteorologist, the arch was estimated to be 101 or 102 miles above the earth. More recently Sir John Herschel estimated that the arch in the aurora of March 9, 1861, was 83 miles above the earth. But he remarks that "the auroral light has been seen below the clouds, as in the polar seas by Parry, Sherer, and Ross, on Jan. 27, 1825; near the chain of the Rocky mountains on Dec. 2, 1850, by Har-disty; and at Alford in Scotland on Feb. 24, 1842, by Farquharson; nay, even habitually seen as if hovering over the Coreen hills in the last-mentioned neighborhood, at a height of from 4,000 to 6,000 miles." Herr Galle, from observations made during the aurora of Feb. 4, 1872, estimates the height of the auroral corona on that occasion at 265 miles above the sea level. Prof. Olmsted's conclusion that the auroral arch is seldom below 70 miles in height or above 160 miles, would thus appear to be negatived.
But probably all such estimates must be abandoned, and "our meteorological catalogues," as Arago advised, " must be disencumbered of a multitude of determinations of height, though due to such great names as Mai-ran, Halley, Kralft, Cavendish, and Dalton." - The extent of the earth's surface over which the same aurora has been visible has sometimes been remarkable. Kamtz mentions that on Jan. 5, 1769, a splendid aurora was seen simultaneously in France and in Pennsylvania; and that the remarkable aurora of Jan. 7, 1831, was seen from all parts of central and northern Europe, in Canada, and in the northern parts of the United States. But even these instances, and others of the same kind which might be cited, are surpassed in interest by the circumstance that auroras of great brilliancy occur simultaneously over the major part of both the northern and southern hemispheres. Kiimtz mentions that when Capt. Cook's observations are analyzed, it appears that on every occasion when he observed an aurora australis an aurora borealis had been seen in Europe, or else the agitation of the magnetic needle proved that around the northern magnetic pole an auroral display must have been in progress.
The aurora of Feb. 4, 1872, was seen not only in America and Europe, and over the northern hemisphere generally, as far S. as lat. 14° N., but in Mauritius, in South Africa, in Australia, and probably over the greater part of the southern hemisphere (for Mauritius is much further north than southern auroras are ordinarily seen). - Mairan and Cassini were the first to point out that auroras do not occur at all times with equal frequency or in equal splendor. The former mentions that a great number of auroras were seen at the beginning of the 16th century (a misprint probably for the 17th, as the context seems to imply) to beyond the year 1624, after which nothing more was heard of them till 1686. Kamtz mentions that between 1707 and 1790 there was a remarkable increase followed by decrease of auroral action, the maximum frequency being attained in 1790. Prof. Olmsted considered that there was sufficient evidence to establish a period of 20 years during which auroral displays are frequent, preceded and followed by intervals of from 60 to 65 years during which few are witnessed. But it is open to question whether the existence of this long period is as yet established.
The actual frequency of auroras cannot be inferred from observations made in temperate latitudes, where alone hitherto any attempt has been made to determine long periods. The longest period which has been thoroughly established is one of about 11 years. This period is associated with the occurrence of magnetic disturbances in cycles of 11 years. The connection between auroral action and disturbances of the earth's magnetism appears to have been demonstrated, though doubt still remains as to the exact nature of the association. The perturbations of the magnetic needle undoubtedly attain their maximum extent at intervals separated by about 11 years. The researches of Sabine, La-mont, and Wolf appear to have established that fact beyond dispute. Hence we may infer that the auroral action waxes and wanes within the same period. - A remarkable association also appears to exist between disturbances of the earth's magnetism and the occurrence of spots on the sun. It has been demonstrated that the solar spots increase and diminish in a period of about 11 years; and that this periodicity corresponds exactly with the periodicity of the magnetic perturbations.
A great solar outburst witnessed by Carrington and Hodgson, Sept. 29, 1859, was not only accompanied by extensive magnetic disturbances, but on the same day remarkable auroras occurred in both hemispheres. Telegraphic communication was interrupted on all the principal lines; the operators at Washington and Philadelphia received sharp electric shocks; and the pen used in Bain's system of telegraphy was followed by a flame. Some doubt has been thrown on the supposed connection between these circumstances and the solar outburst, in consequence of the failure of observers to obtain any corroborative evidence during the past 13 years; but the connection between the condition of the solar surface and the earth's magnetic state, and therefore the connection between the solar spot period and auroral displays, has been thoroughly established. The following table exhibits the number of auroras seen in each month, in America and Europe, according to the observations of Prof. Loomis of Yale college and Kamtz of Germany. These observations, however, must not be looked upon as indicating the relative frequency of auroras in America and Europe, because the observations of Loomis and Kamtz range over a different number of years:
In each case there is a double maximum, the two equinoxes being the epochs at which auroras are most frequent; and it is noteworthy that in these months the solar poles are most inclined toward the earth, the southern pole in March, the northern pole in September; so that the southern spot zone is nearer to the centre of the sun's face in March than at any other time, while the northern spot zone holds a corresponding position in September. - As to the electrical character of the phenomenon no question can be entertained, though there are few problems of greater difficulty than the determination of the exact manner in which the electrical action is excited. It has been held by some that the aurora is due to electrical discharges from the earth. Through some cause the earth, regarded as a vast magnet, becomes overcharged (according to this theory) with electrical energy, and it is as this energy is gradually dissipated that the splendors of the aurora are displayed. It has been noticed that whenever the earth's magnetism is unusually intense an auroral display is to be expected. As soon as the aurora has made its appearance the intensity of the magnetic force begins to diminish.
The more brilliant the aurora, the more rapidly is the extra energy of the earth's magnetism dissipated. "It has also been observed by operators of the Bain or chemical telegraph, that very singular effects are produced by the aurora upon the telegraph wires. The atmospheric electricity generated during thunderstorms passes from the wire to the chemically prepared paper, emitting a bright spark and a sound like the snapping of a pistol. It never remains long upon the wires, though it travels sometimes 40 or 50 miles before discharging itself. But the electricity produced by the aurora passes along the wires in a continuous stream with no sudden discharge, effecting the same result as that by the galvanic battery. A colored mark upon the paper is made by the positive current of the aurora as by the positive pole of the battery; the negative current, on the contrary, produces a bleaching effect. Preceding the appearance of the aurora faint blue lines appear on the paper, which gradually become stronger and darker so as to burn through several thicknesses of it.
The effect then disappears, and is soon followed by the bleaching process, which entirely overcomes the artificial current of the batteries. "When these effects have been observed, the aurora follows, and presents some of its most beautiful displays along the lines of these telegraphs; and so familiar have the operators become with the disturbance which the aurora causes, that they can predict its appearance with much certainty. They regard the electricity generated by it as precisely that of the electro-galvanic battery, which is distinguished by its voluminous current without intensity of action, differing from atmospheric electricity or the kind developed by friction, which may be dissipated by placing a wire conductor leading to the ground in close proximity to the line of wires." Capt. McClintock observed in the arctic regions that the aurora was never visible above ice fields, but that whenever an aurora was in progress the light appeared always to be gathered over the surface of the open water. Water being, as is well known, an excellent conductor of electricity, while ice is a non-conductor, we may infer that the peculiarity observed by McClintock was due to this difference in the conducting powers of ice and water.
In fact, on the theory that the aurora is due to electrical discharges from the earth, these discharges were interrupted by the fields of ice. - The study of the aurora with the spectroscope has revealed some important facts, though it has as yet thrown no light on the nature of the phenomenon. Angstrom of Sweden, in the winter of 1867-'8, recognized the existence of a bright yellow-green line in the auroral spectrum; and Otto Struve of Russia presently confirmed this result. It was at the time supposed that this line constituted the whole of the spectrum; and Dr. Huggins, commenting on the discovery, remarked in 1868 that the result seemed surprising when the ordinarily ruddy hue of the aurora was taken into account. "But Gen. Sabine tells me," he adds, "that in his polar expeditions he has frequently seen the aurora tinged with green, and this appearance corresponds with the position of the line seen by M. Struve." Later observations, however, and especially those made by Prof. Winlock in this country, have shown that the auroral spectrum is far more complex than had been supposed, and that it is also variable. It would appear that the bright green line is always present, and that it is nearly always the brightest line of the spectrum.
But there is also a band in the red which, though usually much less intense, yet becomes even brighter than the yellow-green line when the red streamers of the aurora are exceptionally brilliant. The wave lengths of the green and red light correspond respectively to 558 and 035. Besides these there are faint greenish and bluish lines corresponding to wave lengths 544, 531, 522, 518, 501, and 485. Two other bands in the blue and violet between the lines F and G (one of them very close to G) have been detected in the spectrum of white parts of the aurora. They disappear or become faint in the parts having an intense red tint. During the great auroral display of Feb. 4,1872, Father Perry of the Stonyhurst observatory (England) remarked that "the green line could always be detected, even where the unassisted eye failed to notice any trace of auroral light. This," he adds, "might suggest the advisability of a daily observation with a small hand spectroscope for those who are desirous of forming a complete list of auroral phenomena.
Magnetic disturbances are a sure guide in the case of grand manifestations of aurora; but might not a very slight aurora be observable without the magnetic needle being sensibly affected? " One of the most remarkable circumstances hitherto ascertained respecting the aurora is the partial agreement of its spectrum with that of the solar corona. It is not indeed the case, as is sometimes stated, that the principal line in the coronal spectrum (known as the 1474 line, because agreeing with the corresponding line of Ivirchhotf's scale) coincides with the bright yellow-green auroral line; but another and fainter auroral line agrees with Kirchhoff's 1474, and there is sufficient general resemblance between the coronal and auroral spectra to justify the theory that a real resemblance exists between the aurora and the solar corona. This theory was first worked out and published by Prof. W. A. Norton of Yale college; but Prof. Winlock of Cambridge also formed and published a similar theory. - Some doubt seems still to prevail on the question whether the bright green line of the auroral spectrum belongs also to the spectrum of the zodiacal light.
Angstrom and Respighi have asserted that this is the case; but others deny that the auroral green line is ever seen in the zodiacal spectrum save when an aurora is in progress. Mr. Webb observes of the zodiacal light, Feb. 2, 1872: "It seemed to show a ruddy tinge not unlike the commencement of a crimson aurora borealis; this may have been a deception, but it was certainly redder or yellower than the galaxy. At 7 I examined it with a pocket spectroscope which shows very distinctly the greenish band of the aurora; but nothing of the kind was visible, nor could anything be traced beyond a slight increase of general light, which in closing the slit was extinguished long before the auroral band would have become imperceptible." M. Liais also, who has for several years studied the zodiacal light in tropical countries, finds its spectrum to be ordinarily continuous. Yet undoubtedly the yellow-green line is seen in the spectrum received from the region occupied by the zodiacal, during auroral displays; though whether it is then simply the auroral line seen in the direction of the zodiacal as well as in others, or partly received from the zodiacal itself, remains an open question.
In the latter case it would follow, of course, that there is an intimate connection, as Mairan long ago suspected, between the zodiacal light, which is undoubtedly a cosmical phenomenon, and the aurora, which is as undoubtedly a terrestrial manifesta-tion, though not improbably of cosmical origin. Prof. Olmsted had several years ago assigned to the aurora an interplanetary origin. "The nebulous matter," he reasoned, "like that which furnishes the material of the meteoric showers or the zodiacal light, and is known to exist in the interplanetary spaces, is probably the cause of the auroral displays. The periodical return of the phenomena indicates such a position; so too its rapid motion, which exceeds that of light or electricity, and the extent of surface over which the phenomenon is seen at the same time." It should be added that during the months of January, February, and March, 1872, when auroras occurred with unusual frequency, the zodiacal light shone with exceptional brilliancy.