Chapter XIII: The Solar Cycle

Solar periodicity is a most complex phenomenon. The more it is studied, the less it seems to be understood. Its effects branch out into endless entangled and obscure _fibres_ of fact, to trace all of which back to their root-source would be an almost superhuman task. At present they can only be dealt with in groups and tentatively. Their subtle, and often disguised relationships need much patience for unravelment. They need, above all, a free mind. Prepossessions are sure to compromise truth.

The sun is subject to a rhythmical tide of disturbance, ebbing and flowing in about eleven years. But the flow is irregular and spasmodic. Both the intensity of the crises and the intervals at which they recur vary largely and unaccountably. Probably the eleven-year cycle is involved in others. One, there is reason to believe, brings about alternate accentuations and partial effacements of change comprised within a term of some sixty-five years.[295] And minor pulsations—wavelets on the great rollers—are besides evident. Prediction, nevertheless, remains at fault. Spot-maxima are delayed or anticipated, they are languid or energetic, as the outcome of modes of action defying calculation. Not even the loose fetters of an arbitrary formula have ever been forged for them. The attempt would indeed be hopeless, since the laws governing them, besides being highly intricate in themselves, are plainly disturbed in their working. Circumstances intervene which we must call “accidental.” Could we describe them in detail the science of solar physics would lie before us as an open book.

The “error” of the spot-period may amount to nearly half its normal length. Thus sixteen years elapsed between the maximum of 1788 and the next certainly ensuing, and only 7·3 years separated the culminating points in 1829·9 and 1837·2. A characteristic feature of the representative curve is that it mounts more rapidly than it descends. Maxima succeed minima, on an average, after 4·5 years, while the corresponding minima are only reached after 6·1 years. Substantially, a disparity of this kind is probably always present, although now and again masked by the prominence of a secondary maximum. These peculiarities deserve the most careful attention, as sufficing in themselves to place the sun in the category of variable stars. His spot-curve might almost be said to be modelled on the light-curves of such objects; and the analogy is eminently instructive. We learn from it, for instance, that a spotted condition in the sun matches a phase of strong luminosity in the stars; and are hence led to infer that the sun radiates most powerfully when his disc is most maculated. The extreme difficulty of obtaining direct proof of this relation lends especial value to the side-wind of evidence thus brought to bear.

FIG. 17.—Spoerer’s Curves of Sun-spot Latitude (from _The Sun_, by C.
A. Young).
]

Now an increase of radiation involves a quickening of the sun’s internal circulation; and the process, when hurried, is likely to become tumultuous. So that a connection is easily traceable between heightened brilliancy and photospheric laceration. Spots, however, are only the most conspicuous symptoms of an agitated state. Faculæ and prominences follow suit. They are indeed too intimately bound up with spot-economy to do otherwise. The corona, by a less obvious necessity, sympathises, and the periodicity of all these formations has a double aspect. They fluctuate in mode of distribution as well as in vigour of development. Spots, faculæ, prominences, and corona, all vary similarly and simultaneously in heliographic latitude as the waves of disturbance rise and fall. The spot-zones are not stationary. They shift over the body of the sun according to a definite law enounced by Carrington in 1859, and confirmed by Spoerer in 1861.[296] At maxima they occupy mean positions in about fifteen degrees of north and south latitude; then, as the cycle advances, they close together, and the commotion finally dies out near the equator. Meanwhile, the start of a new series in high latitudes has anticipated the termination of the old. Feeble at the commencement, it gains strength as it departs more and more from its native regions. “Mobilitate viget, viresque acquirit eundo.” Fig. 17, copied from a diagram of Spoerer’s, illustrates the nature of this progression. The overlapping of the curves at minimum brings before us the remarkable circumstance that, as a consequence of successive disturbances breaking out before those antecedent to them have expired, the full duration of each is, not eleven, but twelve to fourteen years. Moreover, spot-production at minima, however inactive, has a twofold nidus; two widely separated zones in each hemisphere are appropriated to it. A further characteristic of the cyclical decline in latitude is that it proceeds in “waves.” “Every fourth or fifth rotation,” Mr. Maunder tells us, “there will be an effort to reach a higher level, a lift of one or two degrees, and then a gradual slipping back until a fresh effort brings another small lift, but a weaker one than the last. And so the cycle goes on; the decline is continual on the whole, but is broken and interrupted by these frequent little struggles to get back to a higher plane.”[297] The growth in spotted area accompanying the descent of the zones is similarly rhythmical. So closely connected, indeed, are these two modes of periodicity that irregularities in the cyclical progression frequently show under the double aspect of abnormal outbreaks of spots, and abnormal movements in latitude. Theories are accordingly valueless that fail to rationalise simultaneously both kinds of facts. Several, indeed, profess to do so, but by constrained expedients.

The zonal law applies, with qualifications, to faculæ and prominences. Eruptive prominences are strictly governed by it. They frequent the spot-belts, it may be said, exclusively. The quiescent kind, on the contrary, avoid them,[298] and have their main gathering-grounds within fifteen degrees of either pole.[299] They may even occur, near spot-maxima, right up to 90° of latitude. As activity decreases, however, they too move downward, and crowd more or less closely towards the equator, although maintaining at all times a wider range than spots. Faculæ show a divided allegiance. They attend on spots, and their principal maxima are hence displaced with the spot-zones, while their affinity with quiet prominences is evinced by the occurrence of secondary maxima in high latitudes. Not that their local arrangement is the same with that of prominences.[300] Discrepancies are frequently noted; and they are important as indications that the two varieties of outgrowth do not originate under identical conditions.

The succession of coronal types is in clear accord with the law of zones. Streamers and prominences march, on the whole, closely together. They unanimously quit the poles after each maximum; they linger in company over middle latitudes, where “synclinals” overarch red flames at epochs of medium activity; finally, they descend towards the equator, the white wings of the minimum corona meeting and spreading above the last members of each decadent eruptive series.

The spectral periodicity of the sun is less marked than might have been expected. It is, indeed, almost confined to spots. The ordinary spot-spectrum at minimum (to repeat what has been already stated) includes many broadened iron lines, replaced at maximum by vanadium and scandium absorption. The nature of the individual spot, however (as we have seen), not the stage of the cycle, is really the determining cause of this diversity, which recurs periodically, simply because minimum-formations are usually of the tranquil sort. The chromospheric spectrum has quite other relations. It does not vary fundamentally; but the metallic rays temporarily added to it become fewer as metallic injections fall off. Nor is the quality of coronal light subject to radical change. Only the relative strength of its constituents slightly fluctuates. In the “winged” type the gaseous emissions are feebler than in the “radiated” type; yet they are always present and always the same.

The virtual invariability of the Fraunhofer spectrum is more surprising, since the reversing strata are in immediate contact with the periodically agitated photosphere. They preserve, nevertheless, an almost inviolable tranquillity, and their composition remains unaltered from one cycle’s end to the next. A single recurrent modification is, however, just traceable. It is that produced by the emergence, at maximum, of the facular bright lines H and K. _Pro tanto_ and _pro tempore_, the symptom constitutes the sun a “bright line star.” Analogous detections in stellar spectra would afford a possibility of determining the spot-periods of globes in the solar condition; but they are, for the present, scarcely to be hoped for. The observation is difficult in the sun; in a star, unless facilitated by extraordinary facular development, it would be impossible.

The throbbings of solar agitation affect his entire system. In how many ways, and by what hidden means, we can but vaguely surmise. Terrestrial meteorology, as a whole, is certainly embraced in the great cycle, although the details of its conformity baffle, by their intricacy, the most painstaking pursuit. Only in the magnetic department there is no room for doubt. A thoroughly satisfactory discussion of the subject was completed in 1898–99 by Mr. William Ellis,[301] who for long years controlled this branch of work at Greenwich. Comparing the observations of the diurnal range of magnetic declination and horizontal force made at the Royal Observatory during the years 1841 to 1896, with the sun-spot numbers for the same interval determined by Wolf of Zürich, he found between the two orders of phenomenon, not only a general parallelism, but a correspondence in irregularities of period and secondary variations of intensity. This is strikingly evident in Fig. 18, copied by kind permission from his paper read before the Royal Society, 10th March 1898.

FIG. 18.—Curves of Sun-spot Frequency and Magnetic Agitation (Ellis).
]

The sympathetic relation extends to auroræ. They even obey a “law of zones” similar to that regulating the distribution of sun-spots. The earth is circled—presumably in each hemisphere—by an auroral belt, which advances into temperate latitudes at epochs of cosmic disturbance, but retires towards the pole as it quiets down. The reality of the connection was singularly affirmed by the simultaneous dearth of sun-spots and auroræ during the seventeenth century. A prolonged solar calm appears to have set in about 1643. Galileo and Scheiner had been at no loss for objects of study; but the diligence of their successors, although unrelaxed, went mostly unrequited. To Cassini, Flamsteed, Hooke, De la Hire, the occurrence of a spot was an event of rare interest, which rewarded perhaps a decade of fruitless watching. Yet, as Mr. Maunder says,[302] the cycle was “submerged” rather than actually abolished, “the crests of a sunken spot curve” being marked by the solitary spots perceived in 1660, 1671, 1684, 1695, and 1705. Definitively, the protracted minimum came to an end in 1716, and there was a normal maximum in 1718. Meantime auroræ too were in abeyance.[303] None were seen in England from 1575 to 1706, when a glimmer of polar lights heralded the magnificent display witnessed by Halley, 17th March 1716. That there was concomitant magnetic quiescence need not be doubted; but Gauss of Göttingen was still in the distant future, and nothing could be known on the subject.

Individual outbreaks on the sun are often unmistakably associated with commotions of the terrestrial magnetic system. These so-called “storms” are world-wide in their nature, abrupt in their origin, and bear witness to some sudden _vital_ spasm attacking the globe as a whole, and at once. Auroras and earth-currents make part of these mysterious affections, which commonly reach their height when a large spot-group is nearly central on the disc—that is to say, when it is broadside on to the earth. Instances abound. On 17th November 1882, the photosphere was, to the naked eye, conspicuously rent. The coincident aurora and magnetic storm were said to “beggar description.”[304] A spindle-shaped beam, which darted that night across the sky, was indeed a unique phenomenon, and, on its farthing-candle scale, recalled the amazing solar flambeau of 1st September 1859. Nor can the contemporaneous twitchings of the Kew magnets on this latter occasion be regarded as accidental, any more than the sudden small disturbance of all three magnetic elements which accompanied an outburst of faculous light on 17th June 1891.[305] The magnetic turmoil raised by the transit of the enormous spot of February 1892 was exceptionally violent. Earth-currents seriously interfered with telephonic and telegraphic communication in all parts of our busy world;[306] the needles at Greenwich went completely off the prepared track of photographic registration;[307] and an auroral pageant completed the programme of response. Similar concurrences were observed in February 1894, September 1896, and March 1898, to mention a few out of a multitude of cases. Yet the sympathetic connection is not invariably manifest. A hole in the sun may evoke no earth-trouble. Mr. Maunder hence concludes that “though sun-spots are the particular solar phenomenon most easily observed, we must not therefore infer that their number and extent afford the truest indication of the changes in the solar activity which produce the perturbations we remark in our magnetic needles.”[308]

Not the spot itself, but the connascent agitation thrills the terrestrial organism. Quiet formations pass unheeded; crises of growth or reconstruction meet with instant rejoinders. Tacchini[309] accordingly holds that _chromospheric_, not _photospheric_ phenomena are, in this respect, truly influential, and that it is to the fierce flame-rushes above spots that the magnetic nerve-system is sensitive. The view is favoured with some hesitancy by Professor Hale.[310] Professor Bigelow, as the result of much suggestive inquiry, affirms that “from the sun to the earth come two great supplies of energy, both types of radiation through the ether”—one propagated by plane waves, the other by vortical rotation—“and possessing very different properties, the one visible to the eye, the other visible to magnetic perceivers.” And he computes, from various indications, a value for the normal magnetism of the sun about one-fifth the maximum of steel, which may, at epochs of extraordinary disturbance, be augmented fifty-fold.[311]

Variations in the sun’s electrical state assuredly accompany his more obvious cyclical changes. “May not,” Dr. Schuster asks,[312] “the periodicity of sun-spots and the connection between two such dissimilar phenomena as spots on the sun and magnetic disturbances on the earth, be due to a periodically recurring increase in the electric conductivity of the parts of space surrounding the sun?”

The surmised alteration may be a consequence, it can hardly be the cause of solar periodicity. Its actual occurrence, however, is far from unlikely. Certain cometary phenomena lend it a qualified support. The grouping together near sun-spot maxima and minima respectively, of the bright and faint apparitions of Encke’s comet during a century (1786–1885), forced itself upon M. Berberich’s attention in 1888.[313] Correspondences of this unexpected form were even traceable with displaced epochs of activity, such as the retarded maximum of 1788, and the premature maximum of 1837. Some indications were besides gathered that comet-discoveries become more numerous as the tide of solar energy rises, many that would otherwise pass unseen being lifted into visibility by accesses of transmitted excitement. But this relation, M. Berberich admitted, might be more apparent than real.

One still more recondite and unaccountable has been lately adverted to by Mr. J. Halm.[314] He contends that the physical condition of the sun reacts perceptibly on the motion of the earth. Variations in its orbital elements, which have hitherto baffled attempts at explanation on gravitational principles, are comprised, he finds, within the “great” spot period of about sixty-five years. The obliquity of the ecliptic, for instance, instead of decreasing uniformly with the time, shows subordinate fluctuations synchronising with the long waves of solar activity. Its shorter waves, on the other hand, prove to be influential upon the variation of latitude. Mr. Halm considers that the deviations of the terrestrial pole conform unmistakably to the eleven-year cycle, with, however, a lag of about one and a half years in the corresponding epochs. “It may,” he writes, “be taken to be clearly established that the radius of the circle described by the pole of instantaneous rotation is greatest at times of sun-spot minima, and smallest at times of maximum displays of solar spots.” This “holds true,” he adds, “for the whole interval of about sixty years now covered by Dr. Chandler’s investigations.” His explanatory hypothesis is both simple and ingenious. It depends upon the large inclination of the earth’s magnetic axis to its axis of figure, combined with alterations, due to solar influence, in the total magnetism of our planet. Molecular strains along the magnetic axis would—it is plausibly assumed—occasion bodily distortions of the globe, whence should result displacements of the axis of figure relative to the axis of rotation. “The outcome of this hypothesis would” then “be that changes in the state of solar activity, since they produce a measurable effect on the terrestrial magnetic forces, should also be accompanied by corresponding changes in the motion of the earth’s axis.”

The machinery by which electro-magnetic impulses are propagated from the sun to the earth, completely evades scrutiny. Sundry conjectures on the subject have been hazarded, but none of them rest on any sure basis. What we know about modes of communication is chiefly negative. Thus, Hertzian vibrations are not transmitted to the earth with sunlight. They do not, at least, reach its surface. Wilsing and Scheiner tried for them in vain with an electric “bridge” and a galvanometer.[315] They might, indeed, as the experimenters noted, be atmospherically arrested. All that seems certain is that direct magnetic action is concerned in producing the observed perturbations of the terrestrial magnetic system, which are not explicable like ordinary meteorological phenomena as effects of thermal vicissitudes, or convective air currents.[316] They stand apart, and imply special conditions which cannot, without detriment to science, be ignored.

Little progress has been made towards ascertaining the cause of solar periodicity. We are only assured that it is not imposed from without, but arises from within; it resembles a “free,” rather than a “forced vibration.” This conclusion, it is true, tends to relegate the matter to obscurity; for the interior of the sun is a _terra incognita_, and seems likely to remain so. His cyclical changes may belong to his original constitution; they may date from nebular times, and be as inherent as the tone of a bell. Or they may simply characterise a stage of growth, and prove liable to modification and effacement. The study of variable stars will perhaps help to guide our ideas as to the probabilities of the case.

Its full bearings, meanwhile, can only be conjectured. The scope of the disturbance needs to be defined. There are still many open questions connected with it. Does the “smoke-veil” absorption vary with the abundance of spots? Are the temperature of the photosphere, and the depth of the chromosphere affected by it? Can periodical changes of pressure in the reversing layer be detected? These are among the problems of the immediate future. They are already within reach of attack.