Once a Week (magazine)/Series 1/Volume 3/Auroras
AURORAS.
In primitive ages mystery alarmed. Knowledge of his insignificance amid the vastness of the universe inclined man to regard with superstitious awe the invisible but all-pervading forces of which he was vaguely conscious. Attributing to nature sympathy with his fortunes, he conceived that all phenomena had a direct relation to himself—that a mysterious connection existed between the events of his ephemeral life and the cyclical movements of the stars, and he uneasily sought in the complex changes of the heavens for indications of the future that might determine his faltering steps. From its weird and fantastic character the polar aurora is peculiarly adapted to elicit these emotions; and, as that seen by imagination is but the shadow of the actual projected into infinite space; so, in those ages of blood and havoc, the auroral coruscations, shaped by fearful fancy into aerial hosts contending with glittering arms, were conceived to portend proximate slaughters, which, from the spirit of the age, no sign from heaven was needed to presage. These phenomena no longer alarm us; and yet, beyond ingenious conjecture, modern science has made indifferent use of the materials accumulated by observation towards determining the real meaning or origin of auroras. Under these circumstances, the attention of the public having been attracted to late displays of singular brilliancy, and to the remarkable influence these have exercised upon telegraphic lines, some remarks may be acceptable on a subject scientifically so interesting, and, as affecting the chief means of international communication, so important to the welfare of our race.
After briefly reviewing the auroral phenomena, we propose inquiring what special conditions of the earth, atmosphere, or cosmos, ascertained to coincide with their occurrence, may be conceived to have a positive relation therewith either as cause or effect. Certes, coincidences do not in themselves constitute proofs of connection; but, when constantly recurrent, they justify a presumption to that effect, are fairly entitled to a valuation, and may possibly guide our efforts to discover the law they intelligibly suggest.
That which is specially perplexing in the aurora is the irregularity of its appearance. From earliest antiquity down to the present time it has been seen at unequal intervals, yet no period has been assigned to it, nor has anything been determined as to its law. The unknown writer of the book of Job speaks of the “Brightness that cometh out of the North.” Aristotle has recorded the phenomenon, and various other classical writers incidentally allude to it; but that it was then rare may be presumed both from the awe it inspired, and from the very position of that region whereto early science was restricted. To come to later times: in Sweden and the north of Europe it was also rare previous to the eighteenth century, and there seem to have been long intervals without any auroral appearances in England, though a lack of meteorological observations does not absolutely prove the absence of phenomena in an age indifferent to science, and inclined to prefer the comfort of repose to learned vigils. Of later years auroras have been remarkably vivid and frequent, even in places hitherto unvisited by them, for the great aurora of 1859 was the first ever observed in Jamaica since the discovery of that island.
It may be stated generally that auroras increase in frequency with proximity to the poles, but they are seen alike in the frosty winter of polar regions, and the autumn of more genial climes, the atmospheric serenity of those seasons being specially favourable to their visibility, and perchance to their occurrence. An aurora commences after sunset, rarely later than midnight, its duration varying from a few hours to several successive nights, while so manifold are its aspects and so rapid its transitions that they can scarcely be comprehended in a general description necessarily terse.
An aurora is always preceded by the appearance on the horizon of a brown haze, passing into violet, through which the stars may be dimly seen, which is diffused laterally and upward to a height of from 5° to 10°, at which it is bounded by a luminous arc. This is occasionally agitated for hours by a tremulous movement and seeming effervescence ere rays of light rush from it upward into the zenith, glowing with the prismatic colours between violet and purple red, whose rapid undulatory motion causes a continuous change in their form and splendour. Sometimes these columns of light are mingled with dark rays, somewhat like Fraunhofer’s lines in the solar spectrum; at others the whole heaven is radiant with coruscations, whose brilliancy seems intensified by the rapidity of their emission, though it is ever greatest at the arc in which they originate. When these streams of glory, rising simultaneously from various points, unite in the zenith, they form a brilliant crown of light; but this is rare, and always premonitory of the end of the aurora, which then rapidly pales and vanishes, leaving as records of its presence only a faint haze on the horizon, and a few nebulous spots arranged in streaks upon the sky. A faint sulphurous odour is at times apprehended, similar to that attendant on a thunderstorm, and a sharp crepitation has been heard, regarding which the incredulity of some in opposition to reliable testimonies is not very philosophical. Burns, who was a good observer of nature, alluded to it, and his evidence is not to be despised:
“The cauld blue North was flashing forth
Her lights wi’ hissing eerie din—”
Signs of positive electricity have also been frequently observed in the atmosphere at these times.
It has been observed that auroras are most vivid and frequent when the higher atmosphere contains those delicate flowing clouds, termed cirri. These have a singular tendency to Polar arrangement, like that of the auroral rays, and occasionally a train of cirri thus disposed have been identified as having been luminous rays the preceding night,—the vehicle of an evanescent splendour.
The condition of the atmosphere, indicated by cirri, is attended with magnetic disturbances. This having been stated, the coincidence of cirri with auroras gives a special significance to their meridional direction and evolution of light at the Poles, did those facts stand alone. But of all phenomena accompanying the aurora those most invariable are magnetic ones. The needle is deflected by it first west, then east. This is noticed even in distant places where the aurora is not visible, proving that the action is not merely local; and so invariable are these magnetic disturbances, that the celebrated Arago was thereby enabled to detect the presence of an aurora from the subterranean chambers of the Paris observatory.
But the most remarkable evidence of the immediate presence of the aurora is its influence on telegraphic lines, consisting not merely in a momentary interruption of communication like that occurring during a thunder-storm, but in the magnetic action on the magnets and actual occupation of the wires. These strange phenomena vary with the intensity of the aurora, but they have been satisfactorily determined by repeated observation, all telegraphic operations being sometimes stopped for hours.
To apprehend clearly the nature of this auroral action on the wires as distinguished from that of a thunderstorm, it must be premised that the voltaic or chemical electricity used for telegraphic purposes is of low tension, continuous flow, and perfectly controllable; whereas the free electricity of the atmosphere is of high tension, exploding with vivid light when it finds a conductor, and “dying in the very moment of its birth.” During a vivid aurora a new mode of electricity, of totally distinct character from either of these, is revealed: it has low tension, chemical decomposing power, alternating polarity, induces magnetism, and produces on the electro-magnets of a line the same effect as that of continuously opening and closing the circuit. An instance of this specific action may be adduced.
In 1852, when auroras were very brilliant throughout North America, the auroral current manifested itself unmistakeably on many of the telegraphic lines. The main wire of one particular line, to which we have reference, was connected with a chemically prepared paper on a disk, and on this the ordinary atmospheric currents were actually self-registered. The usual voltaic current—decomposing the salts of the paper and uniting with the iron point of the pen—left a blue mark varying with the intensity of its action. On this occasion, the batteries being at the time detached, a dark blue line appeared on the moistened paper, and was succeeded by an intense flame which burnt through twelve thicknesses. This current then gradually died away, and was followed by a negative one which bleached and changed similarly into flame. The force which had thus intervened on the wires continued to act as long as the aurora lasted, and effectually put a stop to business.
Extraordinary as it may seem, the auroral current—the presence of which has been thus made visible—has been actually used for the transmission of human thought very recently.
The brilliant auroras of last autumn, which excited the admiration of England, while interrupting its means of communication, were not merely local, but prevailed simultaneously all over Europe, Northern Africa, Northern America, the West Indies, and Australia, satisfactorily establishing the unity of the action. This magnetic or auroral storm had rendered all the telegraphic lines of Canada and the Northern States unavailable, except at irregular intervals, for several days.
On the 2nd of September, the auroral influence being very active in the Boston terminus of the Boston and Portland line, the proper voltaic current being alternately intensified and neutralised by it visibly, it occurred to the interested operator in the office, that if the batteries were detached from the line, and the wires connected with the earth, the intruding auroral current might, perchance, be made use of. The idea is characteristically American in its utilitarianism. Having communicated this design to his Portland correspondent, the conception was immediately acted on, with fortunate success, and despatches were transmitted for two hours in that manner more effectually than could then have been done with the customary batteries.
A like extraordinary application was made of the auroral current, on the same day, on the Fall River and South Braintree Line.
To a correct apprehension of this strange occurrence it is necessary to remember that the direction of the poles of the several batteries on a line is immaterial, provided it be uniform, otherwise the currents would neutralise each other. When the aurora supervenes on a line, following in successive and differently polarised waves, the ordinary voltaic current is alternately neutralised and intensified beyond control. In the above cases—the batteries having been detached—the abnormal positive current would not increase, or the negative one decrease, the availability of the wires. The waves were observed to endure about fifteen seconds, intensifying with the time, to be succeeded by one of the reverse polarity. The singular phenomena indicating disturbances of the equilibrium of the earth’s magnetic forces have been collectively classed by Humboldt as magnetic storms. They are marked, as we have seen, by cirrous disposition of the clouds, perturbations of the needle, obsession of telegraph-wires, and the aurora. The evolution of light in the latter invariably terminates the movement, as in a thunderstorm lightning re-establishes the equilibrium of the atmospheric electric forces.
After these illustrations of the phenomena attendant on the aurora, some attention may be directed to an inquiry into its causes.
Whatever may be its origin, that the auroral action takes place within the limits of the atmosphere, scarcely higher than the region of cirri, and that it participates” in the movement of the earth, appears from the fact that the diurnal rotation, at the rate of a thousand miles an hour, effects no perceptible change in its aspect. Its absolute height has been variously estimated: by Euler at thousands of miles, by others as within the cloud region. It has been erroneously conceived that the height might be determined by observation of the corona, which is only an effect of perspective, owing to the convergence of parallel rays; each individual seeing his own aurora, as his own rainbow, from his particular point of view. As the centre of the arc is always in the magnetic meridian, simultaneous observation from two stations on the same meridian, with an interval sufficient to constitute a reliable base, might however effect the desired object.
The accepted theory with scientific men is, that the aurora is an electrical phenomenon occurring in the atmosphere, consisting in the production of a luminous ring with divergent rays, having for its centre the magnetic pole, and its production is supposed to be thus accounted for. The atmosphere and the earth are in opposed electric conditions, the neutralisation of which is effected through the moisture wherewith the lower air is charged. In the Polar regions, whereto the great tropical currents are constantly bearing aqueous vapour, which the cold condenses in the form of haze, this catalysis would most frequently occur. When the positively electric vapour is brought into contact with the negatively electric earth, equilibrium would be effected by a discharge, accompanied in certain states of the atmosphere by the auroral light. This is assumed to be contingent on the presence in the atmosphere of minute icy particles, constituting a haze, which becomes luminous by the electric discharge. Aeronauts have found the atmosphere at great heights, while serene and cloudless, to be pervaded by this transparent haze of which cirri are conceived to consist.
In confirmation of this hypothesis, it has been experimentally shown that when the union of the two electricities is effected in rarified air near the pole of a magnet, a luminous ring is produced which has a rotary motion according to the direction of the discharge. 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. Only when it reaches his zenith could he be in immediate contact with the auroral haze, and then only would the asserted crepitation become audible, which is assumed to be identical in nature with that produced by an electrical machine. The sulphurous odour would be due to the generation of ozone from the oxygen of the air.
Now, though this theory would intelligibly explain the mode of phenomenal manifestation, it may reasonably be objected that, in hypothetising a continuous electric action in the atmosphere, it does not sufficiently account for the ascertained periodicity of auroras by assuming that their visibility and the variation in their intensity are consequent on the condition of the atmosphere. It is discreetly silent as to the mode of induction of this special atmospheric condition; and therefore—assuming their invariable coincidence and connection—as to the efficient cause of auroras.
We humbly conceive that the cause must be sought beyond the atmosphere in the fluctuations of that great solar force, to which is primarily attributable the induction of telluric magnetism, and which must enter as a prime motive in all atmospheric phenomena.
The irregularities of solar action have an intelligible exponent in the phenomenal changes observable on the disk of the sun. Its spots are subject to remarkable variations in form and size, contracting or dilating in unison with the variable vivacity of its constitutional force, and the period of these variations—secular, annual, and diurnal—have been approximately determined.
The direct relation between these oscillations of the solar atmosphere and the intensity or direction of the magnetic forces, as indicated by the needle, long inferred, are now satisfactorily established. From late observations made at Christiania, in Norway, by Hansteen, it has been ascertained that the maximum of magnetic intensity corresponds with the minimum of inclination; and that for both the period of oscillation is 11 days, which is precisely the shorter period assigned by Wolff to the solar spots.
To express these results in less technical language, when the luminous atmosphere of the sun is more equally diffused, indicating the highest energy of that constitutive force pervading, vitalising, and perchance evolving it; then, through the tremulous medium of the intervening ether, the earth thrills responsively with intenser life. This epoch of exceptional magnetic intensity is that specially signalised by auroras, more or less vivid, by atmospheric perturbations, and occasionally by volcanic convulsions.
The remarkable auroras of last autumn have been succeeded by anomalous and unkindly seasons, ominous of coming sorrow, which, if not within the power of man to prevent, he might have been prepared to alleviate, or courageously endure, had he been better able or more willing to “discern the face of the sky,” if not from love of abstract science, from the lower consideration of his material comfort.
Whatever the wilful ignorance of man, since he is rarely entirely deprived of divine guidance, or unillumined by transient gleams of light—obscured and diffracted though it be by the medium through which it is transmitted—might not the vague alarm of antiquity represent a dim and confused apprehension that auroras were symbols of the variable activity of a central force, with the fluctuations in which the condition of the earth, as the abode of human life, was connected?