Popular Science Monthly/Volume 53/August 1898/The Aurora
THE public was startled the other day by the announcement that Tesla had discovered a means and manner of telegraphing through space without wires, and that he had, by eight-foot flashes, so influenced the electricity of the earth that it would be felt all over the globe. This is an important discovery, if true—a revelation, now, that will be a revolution in the world of ideas. It may not be out of place to study these tremendous earth currents in their only visible form—the aurora. The subject certainly merits more than a transient treatment.
One of the latest issues of The International Scientific Series, which has now grown to seventy-five or eighty volumes, is entitled The Aurora, and is written by M. Alfred Angot, honorary meteorologist to the central meteorological office of France.
If one wishes an accurate account of the vicissitudes of starting and supporting this splendid series, and of educating the American public to receive and appreciate it, one has only to read John Fiske's admirable biography of the late E. L. Youmans, the pioneer of the movement not only in this country but abroad. To him are we thus indebted for an introduction to the works of Darwin, Huxley, Spencer, Tyndall, Bain, Balfour Stewart, Maudsley, Jevons, Lockyer, Quatrefages, Luys, Vignoli, Lubbock, Romanes, Ribot, and many others in this country; as the editor of the Popular Science Monthly for years Mr. Youmans was entitled to no small commendation for his capable work; but, more than all, to greater credit for the good he has accomplished in this series than has ever been tendered him.
In the present work we have one of these inimitable special brochures, which appeals to a small circle of readers, but it proves none the less interesting for the general reader. The author begins with the history of the aurora borealis—so called in contradistinction to the aurora australis of the south pole—and cites mentions of it by Aristotle, Cicero, Pliny the naturalist, Seneca, and Gregory of Tours. It is shown by these writers to have been an object of superstitious
dread and terror, even down to the seventeenth century, no one being capable of understanding the nocturnal miracle then. For that matter, only the educated few understand it now. But it is no longer a dragon, a harbinger of woe, a pet terror for homilists and prophets, the Valkyrie of the Norse Eddas, nor a reflection of fire that surrounds the north pole or emanates from a polar cavity or from the interior of the earth at that point—which Nansen possibly just escaped, and Andrée is probably now experiencing! One Danish writer, who called it "the King's Mirror," and wrote a work with that title in 1250, thought the aurora was "produced by the ice which radiates at night the light which it has absorbed by day." Since 1621 it has been called aurora borealis by scientists, or in popular parlance "the northern lights" or "streamers." It is "nordlicht" in German, "nordljus" in Swedish, and "nordlys" in Danish, all meaning northern lights.
The aurora in its most varied and interesting forms is either motionless or rapidly and incessantly scintillating. Of the first class there are three forms: (1) Faint lights without very defined form; (2) more distinct, in patches or clouds; (3) clearly defined arcs touching the horizon at either end. Of the second or moving class there are: (4) irregular arcs, formed of intermittent rays; (5) rays isolated from each other at a greater or less distance, converging to a fixed point in the sky, and sometimes forming around this center a crown or glory; (6) non-homogeneous bands, formed of rays pressed close together, which have not all the same degree of brilliancy; sometimes these bands fold over on themselves, becoming draped auroras, the most beautiful of their manifestations, aside from the boreal crown.
It sometimes happens that these arcs, instead of being circular, are distinctly elliptical; the two curves which form the upper and lower edge of the arc may or may not be parallel; sometimes, instead of a single circular arc, there are two, three, or four arcs, all perfectly concentric. Multiple striped arcs are not uncommon. Stars have been discerned through the rays, as also through the arcs, proving their extremely diaphanous character.
The chief characteristic of the auroral rays is their extreme variability. They are subject to two kinds of movements: the lateral or sidewise, and the longitudinal or upward; both movements are very rapid. A ray in twenty-seven seconds covered a distance of ninety degrees, or half the heaven, in one instance. At times a ray remaining nearly in the same place is seen at the upper extremity to dart toward the zenith or lengthen itself upward. At other times it rises and falls alternately, vibrating, and it is then said to dance. By sixteenth-century writers these rays were called "leaping goats" and "flying fires"; such rays are still known in Canada as "marionettes," and in the Shetlands as "merry dancers."
The direction of all the rays passes, as a rule, close to the magnetic zenith of the locality, or may radiate therefrom, as we have seen, forming a crown; the center of this crown may be either luminous or obscure. At certain moments the rays which compose this crown or glory enter into rapid movement, become very brilliant, and take on, instead of the usual yellowish-white color, vivid tints of red and green. This is one of the finest auroral effects, if not the finest. When one of these crowns forms in the midst of an already existing aurora, all the other lights of the aurora pale, to reappear when the crown is dissipated.
So much for the forms of the aurora. Now as to the colors. The slow-moving forms are white, lined with yellow, milky and difficult to define. Rose carmine is the next most noticeable color. In the rapid-moving arcs, crowns, wreaths and draperies, the center is usually yellowish, one extremity red, the other green. The red is almost always toward the lower part and also in the direction to which the ray moves, while the green is above and behind. For instance, if a ray darts down from a crown, the lower end will be red, the upper green. These colors will be very brilliant; when the red is very brilliant, the green is as intense. The red remains longest and fades last, when fog obliterates the aurora finally.
Our author, it will be seen, is a close observer, and furnishes reasons for all his deductions. He has discovered that the brilliance
of the colors bears a definite relation to the state of the atmosphere. In high latitudes. Sir John Franklin, McClintock, Weyprecht, and others aver that the coloring of the aurora was less strong when the air was very pure, and increased when it became foggy. The fine drapery forms are generally seen where seas are open in winter, free from ice, hence subject to fogs, as in Norway, Spitzbergen, and Newfoundland.
The light from auroras is very feeble; only a few lines of print can be read; while by the light of the full moon this is easy. The intensity of the aurora rarely exceeds the light of the moon in her first quarter, even in the arctic regions; this is corroborated by Parry, Kane, Hayes, Nordenskiold, and others. Auroras are less frequent in the full of the moon, paling in her effulgence, which drowns the auroral display; but some of the brightest have been seen at full moon. Some have said that auroras could be seen in daylight, but this sounds like a ghost story.
It has been almost impossible to photograph the aurora, although in the winter of 1882–'83 a Swede at Spitzbergen claimed to have secured a faint image in eight minutes and a half. This was done before access was had to the orthochromatic plates, color screens, or lightning emulsions of to-day.
Some observers claim that the stars scintillate less when seen through an aurora; but this is caused, according to Montigny, by the presence of fog. On the contrary, it is clearly proved that the scintillation increases during any magnetic disturbance, even when the latter is not accompanied by an aurora.
As to the study of the nature of the light of an aurora there are two methods: by the polariscope and by the spectroscope. By the former it is easy to recognize whether it is a natural light emanating from a self-luminous body, or whether it reaches the eye after undergoing one or more refractions or reflections. Biot, in 1817, in the Shetlands, could not discover the smallest trace of polarization; this has been confirmed by Macquorn, Rankine, and Nordenskiold, and proves that the light of the aurora is not, like that of rainbows and halos, the result of reflection or refraction, but is itself luminous.
This important discovery is confirmed by the spectroscope. If the light emanating from a solid or liquid incandescent body is passed through the spectroscope, the resulting spectrum is continuous; if, on the contrary, the source of the light is gaseous, the spectrum is composed of a certain number of bright lines or stripes separated from each other by dark intervals. The number, the position, and the brilliancy of these bright lines depend upon the chemical constitution of the glowing gaseous body. The spectrum of the aurora was studied by Angstroem, in 1866, for the first time, and is essentially a spectrum of lines, hence gaseous by nature; it can not, therefore, be due to a reflection of the light of the sun, as has been supposed. The spectrum of the aurora runs the gamut from red to yellow, green, blue, and even violet, the last line of the spectrum having been seen but once, by Lemstroem in Lapland. Some of the lines are very similar in position to the spectrum of the electric spark or of lightning. The fourth line has not been found in any known body, and Angstroem attributes it to phosphorescence or fluorescence. For instance, oxygen is phosphorescent, and there is an abundance of ozone created by the aurora. A drop of sulphate of quinine has been made luminous by the action of the rays of the aurora, and so also has the double cyanide of platinum and potassium.
But, although we are getting nearer to the truth as to the nature of the auroral light, it is not yet clearly known. There is also as much dispute as to whether any sound accompanies the aurora. The northern nations believe in the existence of this sound, like the rustling
of silk or of straws, a "crackling" coinciding with the darting of the rays. This also is not well proved. In arctic regions it is probably confounded with the incessant crackling of fields of snow and the faint clicking of small needles of ice in the process of their formation. In 1870, the aëronaut Rollier, escaping from besieged Paris in a balloon, landed in Norway, and he claims to have heard a persistent sound the whole time he was in a certain cloud, accompanied by a strong odor like ozone, very irritating to the bronchial tubes. A fine aurora was observed at precisely that time. Bergmann compared this odor to that of sulphur, and Trevelyan to that of electricity (?).
It is not necessary, at this juncture, to enter into a discussion of the extent, position, or periodicity of auroras, nor their relation to sun spots. But some new facts may be added that are valuable. There is a close resemblance between certain effects of the aurora and cirro-cumulus and cirro-stratus clouds, which are the highest clouds; so that it is difficult to tell whether a given effect is due to real auroral light or to these high clouds lit up by reflected light. Sometimes these bands of clouds stretch out in long parallel lines; again, they are due north and south and lie parallel to the needle of the compass, and are then called "polar bands." It has been discovered that when these polar bands occur during the day an aurora follows that night.
As long ago as 1580 and 1590, Tycho Brahe observed that the appearance of these polar bands and halos coincided with the presence of spots on the sun. Klein, in our own day, comparing twenty-five years of observations made with the greatest care at Cologne by Dr. Garthe, has confirmed the fact that these high cirrus clouds and polar bands follow as to their frequency the same laws as the spots on the sun; they succeed each other or even coexist. Many observers of the aurora are of the opinion that the appearance of the aurora depends on the presence of these clouds in the sky, which are in turn due to magnetic disturbances of the earth's photosphere originating in the sun spots. There are other evidences that the aurora is intimately connected with material particles in the atmosphere, like clouds and fog; for instance, when two or more rays in an aurora cross, the light is augmented, or when an aurora makes a fold on itself as in drapery forms, showing that there are two material "thicknesses" of the substance; moreover, the wind also acts on the aurora, which is torn after a tempest, showing that the wind has acted upon luminous clouds which are part of the aurora; finally, the presence of clouds seems to favor the formation and development of auroras. So there is an intimate relation between them.
The rare phenomenon, St. Elmo's fire (balls of fire alighting on ship masts and spars), is frequent during auroras, but this is about the only form of electric disturbance of the atmosphere occurring during auroras. Andrée, in 1882–'83, at Spitzbergen, with a Mascart electrometer, found that before an aurora appeared the positive electric potential of the air diminished rapidly, and even became negative, as usually happens when it rains; but as soon as the aurora appears the potential takes as before a high positive value. But it must be remembered that the electric potential of the atmosphere, especially at a short distance from the earth, varies constantly in an abrupt and irregular manner; so that it would be imprudent to assume any coincidence, even during an aurora.
Auroras seem to depend for their form and position in space on the distribution of magnetism on the surface of the globe. The appearances of auroras coincide in the majority of cases with magnetic disturbances on the earth's surface. The auroral arc generally has its summit near the magnetic meridian, where the compass needle points. The force which governs the auroras appears to be the same which the magnetic needle obeys. In general, the point where the rays converge in auroral crowns, the center of the crown, is near the magnetic zenith. The variation, in fact, in such instances is but one degree, which may be allowed as the amount of probable error in observations.
The conclusion is, therefore, that the earth's magnetic forces certainly play the most important part in the auroral display. The arcs or "bands" arc nearly perpendicular to the magnetic meridian, and the direction of the rays parallel to the magnetic needle. The deviations from these rules are due to atmospheric conditions of temperature and humidity. The electric discharges which constitute the aurora borealis encounter all sorts of conditions of the atmospheric strata which are unequally conductive; hence lack of symmetry in the auroral displays. Meteorological conditions very likely influence not the production of the aurora, but its form and position. There is an intimate relation, as we have already pointed out, between the weather and these magnetic disturbances.
The magnetic needle sometimes begins to be agitated an hour before the appearance of an aurora. The magnetic disturbances last a long while, often for twenty-four hours. Motionless arcs and faint auroras do not affect the magnetic needle sensibly, while, during an active aurora, which is apparently nearer, the needle is greatly agitated, especially when the great red and green rays flash suddenly like lightning.
Arago has shown that if the aurora seems to be absent during magnetic disturbances, it is often because it is too distant, or below our horizon, and visible only in more northern latitudes. In the arctic and antarctic regions there is small deviation of the magnetic needle, but this is considered due to the great height of these auroras, while local auroras are nearer the earth, of small extent, and due to local disturbances.
Telluric currents, passing along telegraphic wires, communicating with the earth at both ends, are visibly and often violently affected
during an aurora, setting electric call-bells ringing, preventing the transmission of messages by throwing the mechanism out of gear, and often proving a source of positive danger. This is not due to, or during, a storm. The telluric currents cease when one end of the line is isolated; they are only manifested in single lines which have their return to the earth, but they may affect submarine cables, and they vary in direct ratio with the length of the line.
Balfour Stewart, in 1869, compared the earth to the center of a Ruhmkorff's coil, of which the circuit is completed in the higher strata of the atmosphere. Electric movements are hence produced in these higher strata when terrestrial magnetism or the telluric currents undergo rapid variations.
The aurora is supposed to be a purely terrestrial phenomenon, without regard to outside influences. The position of the earth in its orbit has no influence on the movements of the aurora. The movement of the earth from east to west is not the prevailing one of the aurora; the inverse movement is more frequent.
The magnetic theory of the aurora must now yield to the electric theory of its cause. We hope to show in a future paper the intimate relation of magnetism and electricity, but the subject is too vast at this juncture to examine. Halley, in 1716, thought the aurora due to luminous magnetic vapors, but electro-magnetism was then unknown. Dalton, in 1793, and Biot, in 1820, thought it was produced by ferruginous particles in the air, the dust of volcanoes, like that which caused the "red sunsets" several years ago from a volcanic eruption in Java. Van Baumhauer, in 1840, thought auroras due to the fall of cosmic dust, becoming incandescent when it entered the earth's atmosphere, as in the case of meteors and falling stars. Toeppler, as late as 1872, supported this idea, and even supposed the halo around the moon due to the same cause.
The electric theory seems destined to supersede the cosmic, optic, and magnetic theories. Canton, as early as 1753, pointed out the close analogy between auroras and the light of electric discharges produced in very rarefied air; he recognized also the fact that a tube of such air becomes luminous when moved about near a charged conducting body. In his view the aurora was but the form which storms take in polar regions. So far he was very nearly correct.
The ideas of Canton were adopted by Priestley, Eberhard, Frisi, Pontoppidan, Benjamin Franklin, and others, but without making much progress. A very similar opinion was adopted by Fischer in 1834, whose theory was that auroras were caused by electric discharges due to the positive electrization of the atmosphere; these discharges being produced at the moment when the electric equilibrium is re-established between the atmosphere and the earth, by the intermediary of particles of ice floating in the air. This was followed by the theories of Dove, de la Kive, Lemstroem, and others, the latter producing on a Holtz machine electric discharges which electrified rarefied air in Geissler tubes.
Another theory, proposed a few years ago by Edlund, seems nearer the truth than any yet propounded. He began with the phenomena of unipolar induction, discovered by Weber. This is the name given to those currents which arise in each half of a metallic sheath which surrounds a magnet when the sheath is rapidly revolved around the magnet. It is known that the general phenomena of magnetism can be satisfactorily explained on the hypothesis that the earth is a magnet with two poles. The earth rotating on its axis is similar to the sheath, and unipolar action is induced.
Edlund shows that a molecule charged with positive electricity, taken on the surface of the earth, is subjected to two forces: one, from below, driving it upward into the air; the other force, perpendicular to the first, drawing it toward the nearest pole. The first movement is in full force at the equator and nil at the poles; the other is nil at the equator, increases with the latitude, and is again nil at the poles. The tendency is then for the electrified molecule to rise in the atmosphere, thereby accumulating a store of electricity in the higher atmosphere, the movements thence tending toward the poles. As rarefied air and gases are good conductors, being like a vacuum, the electricity will find little resistance at the poles, in returning to the earth. It can either return in disruptive discharges like storms, or in slow discharges in the form of continuous currents, as in the polar aurora. The vertical force is here nil. The electricity of the atmosphere generally re-enters the earth before it reaches the poles, producing local auroras. In polar regions the tension or attraction is much stronger, the flow downward more rapid, having only to overcome the resistance of the air, hence producing greater displays.
Edlund's theory satisfactorily accounts for (1) the direction of the rays of the aurora; (2) the frequency of auroral crowns; (3) the zone of greatest frequency being in polar regions; (4) the deviation of the summit of the auroral arcs from the magnetic meridian (which does not coincide with the geographical meridian); (5) the accidental deviations caused by atmospheric conditions, clouds, humidity, and temperature, that cause curtains, draperies, cirrus effects, etc. This theory seems of all others the most tenable and credible, and should be retained, at any rate for the present, until a more plausible and satisfactory one can scientifically be adopted. It explains clearly the occurrence of polar auroras which are simple, and less reasonably the auroras of immense extent covering two thirds of the globe. These are always accompanied, as we have seen, by marked magnetic disturbance and telluric phenomena, which are probably more active as causes than the Edlund currents. It is natural to suppose that any variation of the earth's electrical equilibrium or potential would tend to draw down the atmospheric currents. Here would come in the action of the electrical currents which coincide
with sun spots, and which undoubtedly produce electric storms in summer and auroras in winter. Magnetic disturbances and telluric currents have a known relation to sun spots; so also must the aurora in its secondary widespread form.
This theory accounts for nearly all the points on which there is now any uncertainty. There only remains the spectroscopic side of the subject, as intimated, to account for all the lines in the spectrum of the aurora. The yellowish-green line (the fourth in the spectrum) to which we have referred is only found in the "zodiacal light" which travels along the zodiac at certain times of our year. But while there is otherwise no identity between the two forms of light, this would indicate an intrastellar or supermundane source of both lights. So it is very probable that cosmic causes, foreign to our globe, may determine on our globe the production of the aurora. We have dwelt in other papers on the influence of the sun spots, so it is not necessary to revert to them, except to confirm our previous theory and impressions. These phenomena all appear to obey the successive phases of solar activity, and the electric theory of the aurora fully accounts for their relation to each other. If sun spots can cause a widespread action of the aurora in polar regions, why can they not induce magnetic disturbances, electric storms, and all the fury of tempests in temperate regions, acting along exactly the same lines of current, only deflected sooner to the earth than the rays of force which reach the earth nearer the poles? It is a very simple question, and the answer self-evident.
Laboratory study of electric discharges in rarefied air and gases has not yet reached a point where the spectrum of the aurora borealis can be artificially produced. A study of the zodiacal light, and of the influence of magnetism on light, will be necessary to reveal this factor of the problem. This opens an immense field of further research in which scientists must still delve.