Chapter VII: Structure and Movements of Sun-Spots

A normal sun-spot consists of a round black “umbra,” garnished with a circumferential “penumbra.” The chief member of the group shown in Fig. 6 is a good example. The ground should be almost white, with the granular texture delicately indicated. The details of such objects, however, are seen much better than they can be photographed even by the consummate art of M. Janssen.

FIG. 6.—Sun-spot photographed by Janssen, April 1, 1894 (from
_Knowledge_, vol. xviii. p. 108).
]

One of their characteristic features is the definite separation of their parts. The umbra does not merge into the penumbra, nor the penumbra into the photosphere; the lines of demarcation are as sharp as the edge of a cascade. Their darkness is indeed accentuated by the enhanced brilliancy of the regions invaded by them. An immense area of disturbance usually surrounds an active spot, and this area of disturbance is also an area of actual elevation. A series of micrometrical measures carried out by M. Sykora of the Charkow observatory in 1895 showed that, as a very general rule, the sun’s diameter is lengthened in the direction of a spot on the limb.[122] The fact is most significant, for it indicates relief of pressure as perhaps the cause, and certainly as an accompaniment of solar outbreaks, and thereby associates them with volcanic explosions. Now a region lifted is, on the sun, a region brightened, the effects of absorption diminishing with the rise of level. Hence the exceptional vividness of the photosphere in a spotted neighbourhood.

Faculæ gain higher altitudes, and are consequently still more lustrous. They are like the summit-ranges of a tableland. Their connection with spots is intimate, but not inseparable. Every spot, it is true, claims a retinue of faculæ; but faculæ exist abundantly where there are no spots. It is a moot point whether spots can come into being apart from preceding facular disturbance. “Which is the forerunner of the other?”[123] is a question not to be answered off-hand. As a rule, the embryo spot has apparent priority. The rule, however, is not invariable, and the priority recorded may often be illusory, faculæ being extremely evasive of observation. Their survival, on the other hand, after the openings enwreathed by them have closed, is obvious and constant. The complex relations of the two kinds of phenomenon must be unravelled before the nature of either can be thoroughly understood.

Photospheric structure is very curiously modified in the penumbræ of spots. The roundish granules of the unbroken surface seem as if drawn out into threads, which lie side by side, pointing radially inward, and overhang the umbra with ragged edges, compared by Dawes to those of an untrimmed straw-thatch. And the eaves of this luminous thatch are its brightest part, possibly because of the crowding together of materials forced into a narrow circular space. The whole effect suggests the subjection of viscid masses to a pulling action emanating from the centre of disturbance, by which they are stretched and _carded_ like wool-flocks.

The umbra of a sun-spot shows a cloudy texture, markedly unlike the streaky aspect of the penumbra. This can be perceived, however, only when the seeing is exceptionally good. The ordinary impression is of uniform and very profound darkness. Contrast, indeed, greatly heightens this effect. The obscurity is only comparative. Mr. Evershed estimates the light-emissions from umbræ as varying from about one-twentieth to one-hundredth those of the dazzling photosphere,[124] and when intersected by the black advancing moon during the progress of a total eclipse they seem dully bright.[125] Yet with differences. They are rarely of the same tint throughout. Dawes perceived in 1852 a “black opening” in the umbra to be a characteristic of all well-developed spots. It can only, however, be discerned visually, and that by the aid of special precautions; the sensitive plate takes no notice of this deeper depth of shadow, which has, accordingly, received somewhat less attention than it deserves. Fortunately eye-and-hand portrayals still continue to be made, and they not infrequently afford valuable records of the Dawes phenomenon. M. de Pereira, a Portuguese observer at the Azores, wrote as follows of a group which came into view 20th April 1895: “The sense of a cavity in this spot is unmistakable, as though the crust of the sun were torn and scratched, and the black, or rather dark, under-skin were visible beneath. Definition on this day was the best I have ever seen, enabling me to make the smallest detail reliable. On the 24th, this same spot showed a conspicuous black hole in what I may call the centre of gravity, a dazzling white bridge crossing it from south to north-east, and a smaller one lying on the northern edge of the abyss, the brims being full of curiously intertwined points of photospheric matter.”[126] The chief member of the splendid group visible in the sun’s southern hemisphere during the last half of February 1894 had also an inner nucleus,[127] and the same feature has been studied in numerous examples by Father Cortie, Mr. Maw, and others. It is commonly associated with the presence of “bridges,” and both belong characteristically to the final stages of active spots. Bridge-building is preliminary to the indraught of luminous matter by which photospheric breaches are closed; it might be compared to the trickle under the dyke that preludes the rush of inundating waters. The process is a remarkable one. From abutments (so to call them) at opposite sides of the umbra, segments of light protrude; then at a given moment they unite with a leap or a flash, and the arch stands complete. A beautiful photograph by Janssen of a spot doubly spanned is reproduced in Fig. 7. But the ground is _altogether too dark_. The surface near the spot was dazzling, likewise the facular masses crossing the nuclei.

FIG. 7.—Photograph of a Bridged Sun-spot, by Janssen (from
_Knowledge_, vol. xiii. p. 74).
]

“On the negative,” Mr. Ranyard wrote in describing it, “the brilliant bridge which stretches across the great spot is seen to break up into a number of distinct elongated masses,” and these are evidently the “rice-grains” of the photosphere laid end to end in single file for suspension above the abyss. This kind of structure is probably always present, although often imperceptible. Exceptional facilities are needed to bring out the finer details in spots. Thus Professor Young tells us that, on the rare occasions when powers of six hundred and upward could be profitably used with the twenty-three-inch Princeton refractor, he succeeded in resolving “the apparently club-like, almost bulbous ends of the penumbral filaments” into “fine sharp-pointed hooks, reminding one of the curling tips of flames, or grass-blades bending over. Ordinarily,” he adds, “they are seen as club-like simply because of their brightness and the irradiation and diffraction effects of moderate-sized object-glasses.”[128]

The connection of “bridges” with “black holes” was tentatively explained in a valuable paper presented by Father Cortie to the Royal Astronomical Society, 11th May 1900. He considers that the latter may be the portions of the umbra left uncovered by “faculous veils,” which, extending from the penumbra, not unfrequently lighten up certain regions of nuclear gloom, leaving others more profoundly dark by contrast. Now the relationship between “veils” and “bridges” is obviously quite close. Both represent luminous invasions, although differently organised and conditioned, and both are heralds of decay. Their kinship is on occasions emphasised by the development of one from the other. Twice at least, in 1865 and 1866, the transformation into “roseate veils” of brilliant arches spanning the umbræ of spots was observed by Father Secchi at Rome.[129] He was quite positive about the colour of these “veils,” which seems to intimate for them a gaseous nature, assimilating them to prominences rather than to faculæ.[130]

Sun-spots are rarely solitary. They ordinarily appear in clusters or processions, consisting of one or two dominating members and many satellites, down to mere umbral dots and penumbral scraps. Individual spots show endless varieties of conformation. The nuclei are often multiple; as many as nine umbræ have been seen within the compass of a single penumbra. Again, they become pear-shaped, or spiral, or caudate, as if through the action of stresses or twisting forces of an unknown character. The penumbra is equally subject to irregularities. It is sometimes a mere torn strip of fringe; half the umbra may be duly furnished with its _valance_, while the other half remains bare; or the umbra and penumbra may be disjoined by intruding photospheric matter. Fig. 8 shows a “fimbriated” spot from a drawing by M. de Pereira, the partially veiled and bridged umbra of which included two conspicuous black holes.

FIG. 8.—Sun-spot drawn by J. de M. Pereira, 18th June 1894.
]

FIG. 9.—Group of Sun-spots drawn by Miss E. Brown, 15th August 1894.
]

In Fig. 9 three “confluent” spots are represented, drawn by the late Miss E. Brown, 15th August 1894. She described the group as covering “a vast extent of surface with a mass of nuclei and penumbra partially connected and very variable in form.” A few hours later the largest nucleus had assumed a helical form, and seemed to be “throwing out feelers like a jelly-fish.”[131] Strong hydrogen incandescence was spectroscopically perceived to be an accompaniment of these rapid changes. The great spot of September 1898 had in its declining stage a nucleus divided by intersecting bridges into three lobes; it assumed on 3rd October the shape of an ace of clubs.[132] Mr. Maw perceived in the same object on 11th September delicate veins of comparative brightness, termed by him “submerged bridges.”[133] Indeed he believes this to be a constant feature of large spots, the umbræ of which, viewed with a suitable eye-piece, appear no longer uniformly dark, but marked with fine traceries in chiaroscuro. These cannot at present be photographed, and their visual detection accentuates the indispensable co-operative functions of the eye and the sensitive plate.

The level of sun-spots has once more become a subject of active debate. Yet it was believed to have been determined once for all in the eighteenth century by the geometrical reasonings of Dr. Wilson. The characteristic perspective effects of depression below the surface were noted by him in a well-developed spot as it circuited the sun’s globe in November 1769, and the saucer-like conformation of all such objects was universally admitted for a hundred years and upwards, notwithstanding frequent failures to verify the due optical consequences of their changing situations. At last, however, this “venerable theory” (as Professor Frost calls it) has been uprooted from the soil of conviction. It is denied by many on geometrical grounds alone; by some on physical grounds as well. Not that it has been finally discarded, but its credit is gravely impaired. Certainly _all_ spots do not follow its prescriptions; probably very few strictly comply with them. Irregularities of form account for a good many of these deviations, but others cannot be so readily explained away. Mr. F. Howlett, in offering to the Royal Astronomical Society, 14th December 1894, three volumes of drawings representing the fruits of thirty-five years of solar scrutiny, declared uncompromisingly that the Wilsonian view must be abandoned.[134] Father Cortie’s examination of them convinced him too that “the phenomena presented by many spots are directly contrary” to the current hypothesis, a mountainous rather than a cavernous structure being, in certain cases, indicated for the umbra.[135] Nor does the umbra usually vanish near the sun’s edge, as it should if it were simply an excavation with sloping sides. It remains, on the contrary, persistently visible, although foreshortened into a black line. This, to be sure, might be a simple consequence of refraction by vapours congested within the cavity. The explanation is tempting, since it would avail to get rid of many anomalies; still it must not be adopted unreservedly. Originally suggested by Proctor,[136] it has been taken into fuller consideration by Mr. East[137] as a means of exit from the difficulties that hamper attempts to conceive rationally of the build of sun-spots.

There is, indeed, pressing need to conciliate opposing evidence. Thus M. Riccò,[138] at Catania, from eleven years’ study of spots in their geometrical aspect, derived results strongly in favour of the Wilsonian hypothesis, computing for twenty-three especially symmetrical formations an average umbral depth of rather more than a thousand kilometers. Yet the discussion of the long series of Stonyhurst drawings led Father Sidgreaves[139] to ascribe to most spots a convex rather than a concave shape. Professor Hale[140] allowed small weight to testimony so contradictory as that regarding the apparent width of the penumbra at various distances from the sun’s limb, but was inclined to consider the advocates of the Wilsonian doctrine as having rather the better of the argument. “In any case,” he added, “they will hardly be ready to admit that the umbra is at a higher level than the penumbra, for it cannot be doubted that the penumbral filaments overlie the umbra, and frequently unite to form bridges extending completely across it.”

One of the most singular details of spot-phenomena is the occasional appearance of a large umbra as a notch on the limb. This implies its projection in a dark mass against the sky, the encroachment upon the bright disc being perhaps only an effect of irradiation. Its inconsistency with a depressed form was pointed out both by Mr. Howlett and by Father Sidgreaves.

Where, then, is truth to be found in this remarkable controversy? How can the jostling facts be reconciled? Compromises have been resorted to. Spots, it is averred, are cloud-like at certain stages of their growth, crateriform at others. Or individual spots belong to one or the other type, according to the circumstances of their origin. But these are subterfuges; let us take a broader view. The concavity, at any rate, of bridged spots is indisputable. The attribution to them of a “mountainous” character would throw the arrangement of their parts into utter confusion. Moreover, De la Rue obtained in 1861, by stereoscopic means, ocular proof of depression in one such object. The experiment might easily and usefully be renewed. A pair of photographs, taken at an interval of twenty-six minutes, gives, through the sun’s rotation, just the right amount of difference in aspect for combination into one picture in relief. The moot question, “concave or convex,” might thus receive a direct answer. Then if, in a long succession of instances, the answer preserved a uniform tenour, it might safely be concluded that anomalous appearances of lifted umbræ in spots seen obliquely are illusory and of purely optical production.

But we cannot even so escape from the entanglements of the subject. It has different bearings, which have all to be taken into account. Spots are very hot relatively to their light, and their thermal radiations are peculiarly conditioned. Professor Frost’s determinations,[141] carried out at Potsdam in 1892, showed that absorption does not take increasing effect upon them with approach to the limb to anything like the extent that it does upon the corresponding radiations from the photosphere. “The reasonable inference from this,” he wrote, “is that the spots are at a higher level than the photosphere, and hence less subject to the absorption of the sun’s atmosphere.” Mr. W. E. Wilson’s[142] more delicate series of observations in 1893–4 showed likewise that “the radiation from the umbræ of spots does not suffer absorption when near the limb in the same manner as a point on the photosphere.” Nevertheless Professor Langley[143] had found in 1874–5 “the decrement of heat in approaching the limb” to be very nearly in the same ratio for photosphere and spots. This flagrant contradiction between results equally authoritative may not be without meaning, since they were obtained at nearly opposite phases of solar activity: Langley’s, three years after a maximum, when it was verging towards stagnation; Frost’s and Wilson’s, during a period of culminating disturbance. The two latter recommended systematic observations throughout an eleven-year cycle for the purpose of investigating the nature of the relationship, and they respectively threw out the alternative suggestions that, during its course, the thermal condition of spots, or the level at which they are formed, may undergo progressive changes.

A more promising explanation was offered by Egon von Oppolzer.[144] Spot-umbræ, he reminded perplexed solar physicists, are surmounted by abnormally hot chromospheric layers, certainly exempt from absorption. Hence the indiscriminate sum of their radiations and those of the underlying spots gains by comparison with those from the simple photosphere, at and near the marginal parts of the disc. Now the flame-stratum develops chiefly above spots of an active type; and spots of an active type predominate at epochs of maximum. This consideration at once removes the discrepancy between Langley’s results on one side, and Frost’s and Wilson’s on the other. In the first series, absorption produced its full and due effects, because quiet spots being presumably in question, no appreciable overlying source of heat was present. In the second and third, the overlying source was so strong as in great measure to efface the gradations of heat-stoppage suffered by the object beneath. The disproportionate thermal power of spots may be similarly accounted for. Our instruments measure, not only their direct radiations, but also those sent out by ignited materials, to some extent enveloping them.

The solar rotation is a subject much too important to be disposed of in a paragraph; it need here only be said that a by-product of its detailed study has been to throw further doubt upon orthodox opinions as to the location of spots. The rates of axial movement deduced by Stratonoff and Wolfer from their progression round the sun appear, at least _primâ facie_, to compel the inference that they are veritably situated at a level higher than that of the photosphere.[145] Yet here again some fallacy is likely to be involved. In view of all these complications it is scarcely to be wondered at that the _Where?_ has almost superseded the _What?_ in recent discussions about solar maculæ. The upshot, so far, seems to be that they are essentially depressions, although depressions very shallow relatively to their superficial extent. Their abnormal geometrical behaviour is due, in part, as Mr. Maunder has suggested, to the cavities being _over-filled_, and the umbræ consequently dome-shaped;[146] in part to the optical elevation into view of bottoms which should, but for refractive action, be concealed by shelving sides. Their radiative irregularities, again, are explicable by the influence of their coronas of hot flames. As to their rotational anomalies, they must stand over for future consideration, with the remark that, to deduce the position of spots from the degree of their conformity to a supposed law of solar rotation, is to attempt the solution of one enigma by proposing another still more arduous.

The movements of sun-spots are of three kinds. There are first those that belong to them collectively, as objects attached to a rotating globe. With these we are not at present concerned. Next, they have individual “proper motions” of transport over that globe. Finally, they show internal movements variously connected with the processes of their development and decay. The last are mostly spiral or circular, and they evidently ensue upon inrushes of photospheric matter. They are sometimes performed round “black nuclei” as centres; and black nuclei are probably, as we have seen, interspaces between obscurely luminous umbral effusions. But the whirling of spots is not systematic or innate; it does not characterise them essentially; it occurs incidentally, and as a result of disturbance. No fixed rules prescribe its mode or direction. Opposite gyrations have been simultaneously observed in different members of the same group of umbræ, and even successively in a single spot. They are executed in other cases intermittently by fits and starts. A revolution is not often completed; the description of large angles is exceptional. Spots cannot then be described, in any true sense, as “solar cyclones”; the vorticose motions occasionally exhibited by them spring from temporary impulses, and cease when the force of these is exhausted.

The _proper_ motions of spots are indicative of much more than has yet been learned from them. Three kinds of influence seem to be effective in producing them; namely, mutual action, action from without, and action from within. After segmentation, in the first place, umbræ repulse one another; they separate with great velocities. They behave like similarly electrified masses, but whether they really are such or not is an open question. In the second place, growing spots in general move rapidly forward. They share the common drift, but with an acceleration often amounting to three or four hundred miles an hour. It seems as if cooled materials, pouring down upon them from above, drove them forward with the added speed due to a wider circle of rotation. If this were actually the fact, however, macular increase and macular advance should always go together; and they are not uncommonly disunited. Processes of extension in spots may even be accompanied by retrogression over the sun’s surface.[147] Indeed, the conduct of these strange objects is governed by no invariable rules. Strong tendencies visibly influence it; yet none that are irresistible. They can be annulled or reversed by countervailing circumstances. Hence the special need for guarded inferences in treating of this subject.

The movements of sun-spots in latitude are not visibly related to their drift in longitude. They are highly irregular, and not often conspicuous. Carrington, however, perceived in them a kind of inchoate method. Spots, according to his generalisation, situated within the solar tropics (so to speak) tend to approach the equator; spots outside the north and south limit of twenty degrees, to depart farther from it. But the exceptions observed are so numerous as sometimes to go near disproving the rule. We have said that spot-movements in longitude bear some marks of being communicated by exterior agencies. Those in latitude, on the contrary, suggest interior action. They are connected, most likely, with the hidden system of circulation prevailing in the body of the sun, and reflect its local perturbations.

A remarkable feature of photospheric commotions was referred to by Father Cortie at the meeting of the Royal Astronomical Society, 11th May 1900.[148] He termed it “alternation” in disturbance. A group of spots generally includes two chief members, posted respectively in the van and rear of the array. These _take it in turns_ to develop. We are reminded of the reciprocal flickerings of the fragments of Biela’s comet. Analogous pulsations, but on a larger scale, manifest themselves in responsive disturbances north and south of the solar equator. Mr. Maunder had already pointed out in 1894 that an “active train” of spots is often “accompanied by a feebler copy of itself a few degrees north or south. An outbreak of the first magnitude,” he continued, “will indeed reproduce itself in several directions.”[149] Somewhat similar correspondences are noticeable between volcanic foci on the earth; yet the inferences they suggest might prove misleading.

The occultation of a spot 107,500 miles across was observed during the solar eclipse of 15th March 1858. Its vast dimensions were, however, exceeded in the same year by those of an object with the “record” diameter of 143,500 miles. An enormous double spot, which appeared in June 1883, covered an area of 2500 million square miles; and the great spot of February 1892, with its dependants and outliers, spread still further afield. But such gigantic formations are rarely stable. Their history is one of tumults and vicissitudes. Comparatively small circular spots possess individually a much more lasting character, although great outbreaks are the longest lived in their successive modifications. The maximum duration so far registered was for a “composite disturbance,” consisting of four very large spot-groups, and thirteen others of smaller dimensions, which appeared seriatim, and in obvious association, on a restricted region of the solar surface.[150] The manifestation continued for 527 days, from 25th September 1891 to 5th March 1893, while the sun completed nearly twenty-one rotations. About two rotation-periods represent, according to Father Cortie, the average life of a spot.

Most of what we know about sun-spots has been learned by a statistical method of inquiry. Nor can such methods be dispensed with in the future. But they do not alone suffice. They must be supplemented and reinforced by _individualisation_. Each notable spot should be studied in itself and in all its relations, singly, specifically, and generically. Efforts should be made to determine its nature, as though it were a solitary specimen. Can it, without doing violence to plain facts, be regarded as an excavation in the photosphere? Or, if apparent inconsistencies with this view be present, are they such as might be due to refraction? The hypothesis can only be tested by confronting it with particular cases, and trying definitely how far it avails to meet their exigencies. Refractive possibilities in the sun have been, until lately, almost ignored; they are now in some quarters vastly exaggerated. Still allowance has to be made for their realisation, in ways perhaps corroborative, rather than subversive of received theories.

There are many other doubts to be set at rest besides those regarding the interpretation of perspective effects. The record, indeed, of no significant structural detail should be omitted; and what detail of these enigmatical objects can be called insignificant? Above all, variations in their parts and features, whether simultaneous, successive, or alternative, claim the closest attention; since the establishment of a course of correlated changes comes very near to the detection of the underlying causal nexus.

The comparison of one spot with another is a natural sequel to the investigation of each spot in itself. Do their peculiarities, it may be asked, depend in any way upon heliographical position? Do they vary periodically? Can certain traits in sun-spots be classed together as inevitably associated, certain others as mutually exclusive? Wider questions, too, suggest themselves as to the place of spots in the general solar economy, and as to the nature of their connection with faculæ, prominences, coronal streamers, and the totality of solar phenomena. Attempts have been made, both by speculative and practical means, to throw light on these obscure topics, but with results not as yet wholly satisfactory. Meantime, additional facts are needed—facts systematically collected, methodically sifted and compared. Isolated observations are rarely of any considerable value in such complex matters. Meaning accrues to them just in proportion as they can be allied to others made in correspondence with them, but under modified conditions. “Correlate and compare” should be the watchword of astrophysicists.