The silent tempest and the visible discharge: How sunspots dictate earth’s magnetic storms

“ Whatever the exact nature of the aurora, it has long been known to be intimately associated with the phenomena of terrestrial magnetism. Whenever a brilliant aurora is visible, the world is sure to be visited with what Humboldt called a magnetic storm—a "storm" which manifests itself to human senses in no way whatsoever except by deflecting the magnetic needle and conjuring with the electric wire. ”

“ These are among the most mysterious phenomena of nature—so subtle and fleeting as to be almost beyond the reach of science. No visible sign do they give of their presence. No clouds darken the heavens; no thunder peals along the sky. Yet strange influences trouble the air. At this very hour, Professor Airy, the Astronomer Royal at the Observatory at Greenwich, reported a magnetic storm of unusual violence. Said a London paper:
"Just when the signals from the Great Eastern ceased, a magnetic storm of singular violence had set in. ”

“ Maunder proved that magnetic "storms" on Earth originate in some way in the Sun. These "storms" are quite irregular (like our storms in the weather) , we should never notice them in ordinary life, but a telegraph clerk finds them a great nuisance; if they are violent and persistent they may make it impossible for him to send or receive his messages. At the time when Carrington and Hodgson noticed that great disturbance on the Sun, of which I spoke early in this lecture, there was a furious magnetic storm on Earth, which was naturally attributed in some way to the solar disturbance. ”

“ Let a thunder-cloud pass over the telegraph wires, either noiselessly or hurling petards in all directions, the line will be affected. The fluid imprisoned in the sky will act by induction on the electricity of the wires which will result in the vibration of the latter, accompanied sometimes by a flash of lightning. These phenomena may cause grave accidents to the telegraph clerks, unless they are on their guard against the treachery of the lightning. These mute discharges happen frequently, but the spark strikes the telegraph wires often, too, as well as the apparatus in the office. ”

“ The presence of such storms is indicated by the oscillations of the magnetized needle, the disturbance of the currents upon the telegraph-wires, and the appearance of the aurora, of which these oscillations and disturbances are, as it were, the forerunners, and which itself puts an end to the storm,—as in electric storms the phenomenon of lightning announces that, the electrical equilibrium, temporarily disturbed, is now restored. ”

“ 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. ”

“ The only fact that has been actually proved to everybody's satisfaction is that, during the time when the most spots are visible on the sun, there are always more magnetic storms and displays of the aurora borealis. ”

“ Magnetic storms prevail more commonly in some years than in others. In those years in which they occur most frequently, it is found that the ordinary oscillations of the magnetic needle are more extensive than usual. Now when these peculiarities had been noticed for many years, it was found that there was an alternate and systematic increase and diminution in the intensity of magnetic action, and that the period of the variation was about eleven years. But at the same time, a diligent observer29 had been recording the appearance of the sun’s face from day to day and from year to year. ”

“ This period covered five complete sun-spot periods, and the approximate synchronism of the maxima and minima, and the general parallelism of the magnetic and sun-spot changes is patent to the eye. Ellis [23] has also applied an analogous method to investigate the relationship between sun-spot frequency and the number of days of magnetic disturbance at Greenwich. A decline in the number of the larger magnetic storms near sun-spot minimum is recognizable, but the application of the method is less successful than in the case of the inequality range. ”

“ The outflow of electrons from the sun never ceases, if we may reason from the fact that the night sky is at all times feebly illuminated by the characteristic light of the aurora. But when sun-spots are numerous, the discharge of electrons is most violent, thus explaining the frequency of brilliant auroras and intense magnetic storms during sunspot maxima. ”

“ When spots are numerous on the Sun’s disc there is great disturbance of the solar surface, accompanied by fierce rushes of intensely heated gases. This solar activity is known to influence terrestrial magnetism by causing a marked oscillation of the magnetic needle, and giving rise to so-called ‘magnetic storms,’ accompanied by magnificent displays of auroræ, with variations in electrical earth-currents. ”

“ Thus the sunspot maximum indicates increased activity at the surface of the sun, which, according to Abbot (1913 and l9132) , is actually sending us increased heat radiation. During the maximum the magnetic condition of the earth is profoundly affected, as evidenced by northern lights, magnetic storms, earth currents, and variations of the earth's magnetic constants. ”

“ During sun-spot activity eruptive prominences rise to heights of many thousands of miles and streams of electrons are shot forth with such force that they may possibly even penetrate deeply into our own atmosphere causing disturbances such as magnetic storms and auroras. ”

“ The power of sunspots to alter light frequencies shows that these solar storm centers function as enormous magnets. Such magnetic fields are able to hurl charged particles from the sunspot craters out through space to the earth's outer atmosphere, where their ionizing influence produces such spectacular auroral displays. ”

“ Although we must confess that the causes of the aurora are very imperfectly explained, we may hope that the rapid progress which the last few years have witnessed in bringing terrestrial magnetism under the domain of cosmical laws may soon be extended to the aurora, and that we shall see in it fresh evidence that the same forces which cause hurricanes in the solar atmosphere thrill sympathetically to the furthest planets of our system in waves, not only of light and heat, but of magnetism and electricity. ”

“ We see the earth’s magnetism responding to the solar influences, not merely in those rhythmic motions which belong to the periodic variations, but in sudden thrills affecting the whole framework of our globe. The magnetic storms which are called into action by such solar disturbances as the one of September, 1859, are, we may feel sure, not peculiar to our own earth. The other planets feel the same influence,—not, perhaps, in exactly the same way, but according to the constitution and physical habitudes which respectively belong to them. ”

“ It must suffice to say that, while it is not probable that our greater magnetic disturbances are caused directly by solar influence, it is very nearly certain that every violent paroxysm upon the sun is distinctly and immediately responded to by our magnetometers. ”

“ These disturbances, during which the magnetic needle is greatly and universally agitated (not in a particular limited locality, but at one and the same instant of time over whole continents, or even over the whole earth) , are found, so far as observation has hitherto extended, to maintain a parallel, both in respect of their frequency of occurrence and intensity in successive years, with the abundance and magnitude of the spots in the same years, too close to be regarded as fortuitous. ”

“ Marlin and his fellow-astronomers discussed this matter of the phenomenon's cause, it was its effects that had begun to claim the attention of the world at large. For that increase of the sun's speed was already making itself felt upon Earth. Even the great storms in the sun's mass, those storms that we call sun-spots, indeed, make themselves felt upon Earth by the intense electrical and magnetic currents of force which they throw forth, causing on Earth electrical storms and auroras and strange weather-changes. ”

“ His brief, shrewd reply to the remark of Lord Kelvin, quoted above, concerning sun-spots and magnetic storms, will find many highly interested readers, and it is made all the more attractive by the vista of possible future discovery which it opens with the suggestion that "it is not perhaps outside the limits of possibility that both the solar and terrestrial disturbance have a common origin in some invasion of power or matter from outer space—that the solar tumult is the brother, and not the father, of our own aurora." ”

“ Perceiving the nature of the connection between terrestrial magnetism and auroras, Balfour Stewart reasoned that we may extend our inquiries and ask, "If the sun's action is able to create a terrestrial aurora, why may he not also create an aurora in his own atmosphere?" ”

“ Meteoric and cometic systems have been visited by the great magnetic wave, and upon the dispersed members of the one and the subtle structure of the other effects even more important may have been produced than those striking phenomena which characterize the progress of the terrestrial or planetary magnetic storms. ”

“ Between the tropics the hour of the day may be known by the direction of the needle as well as by the oscillations of the barometer. It is affected instantly, but transiently, by the northern light.
When the uniform horary motion of the needle is disturbed by a magnetic storm, the perturbation manifests itself simultaneously, in the strictest sense of the word, over hundreds and thousands of miles of sea and land, or propagates itself by degrees in short intervals every where over the earth’s surface. ”

“ A magnetic storm is in progress. But where is the centre of disturbance, and what are the limits of the storm? The answer is remarkable. If the jerking movements observed in places spread over very large regions of the earth—and in some well-authenticated cases over the whole earth—be compared with the local time, it is found that (allowance being made for difference of longitude) they occur precisely at the same instant. The magnetic vibrations thrill in one moment through the whole frame of our earth! ”

“ An electric storm is generally confined to a small space beyond the limits of which the condition of the atmospheric electricity remains unchanged. A magnetic storm, on the other hand, p 193 shows its influence on the course of the needle over large portions of continents, and, as Arago first discovered far from the spot where the evolution of light was visible. ”

“ And I may notice in passing that two very interesting conclusions follow from this peculiarity. First, every magnetic needle over the whole earth must be simultaneously disturbed; and secondly, the auroral streamers which wave across the skies of one country must move synchronously with those which are visible in the skies of another country, even though thousands of miles may separate the two regions. ”

“ The irregular oscillations when violent are called magnetic storms, and it must be noted that auroral display never takes place except during such disturbances, although a large proportion of the most remarkable magnetic storms are unaccompanied by visible auroræ. ”

“ A bright aurora visible over a large part of Europe seems always accompanied by a magnetic storm and earth currents, and the largest magnetic storms and the most conspicuous auroral displays have occurred simultaneously. ”

“ Magnetic storms are of extraordinary magnitude and frequency in that region. They stand in the closest relation to the auroral discharges, and the disturbances arc greater as the motions of the light-streams become more lively, and the prismatic colours become more intense. Quiet regular arches, or ray-motions, have scarcely any action upon the needle. In all disturbances the declination-needle moved towards the east. ”

“ It is not improbable that, as heavily-charged threatening clouds, owing to frequent transitions of the atmospheric electricity to an opposite condition, are not always discharged, accompanied by lightning, so likewise magnetic storms may occasion far-extending disturbances in the horary course of the needle, without there being any positive necessity that the equilibrium of the distribution should be restored by explosion, or by the passage of luminous effusions from one of the poles to the equator, or from pole to pole. ”

“ Of greater importance for ascertaining the true nature of northern lights has been the demonstration of the relation these phenomena hold to terrestrial magnetism, a mysterious power of which our globe is the conductor. Powerful magnetic currents appear suddenly and disappear as quickly, and we are not able to name the cause of their existence.
Like a vast ocean the magnetic force is constantly surging through our globe, here appearing, there receding, but forever seeking to attain its equilibrium. ”

“ 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. ”

“ This belt divides the regions where these luminous arcs are seen principally to the south from [ pg 41 ] those in which they mainly appear on the northern horizon. In the area next the aurora-pole only the smaller, in middle Scandinavia only the larger, more irregularly formed luminous crowns are seen. But in the latter region, as in southern British America, aurora storms and ray and drapery auroras are instead common, and these appear to be nearer the surface of the earth than the arc aurora. ”

“ 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. ”

“ What we see is the auroral corona; and it is almost always seen when a band passes over the magnetic pole. This peculiar phenomenon lasts but a short time—the band now lies on the northern side of the firmament; gradually it sinks, and pales as it sinks; it returns again to the south to change and play as before. So it goes on for hours; the aurora incessantly changes place, form, and intensity. ”

“ The splendid appearance of colored polar light is the act of discharge, the termination of a magnetic storm, as in an electrical storm a development of light — the flash of lightning — indicates the restoration of the disturbed equilibrium in the distribution of the electricity. ”