Chapter IX: Faculæ and Prominences

Faculæ are outgrowths from the photosphere. This is known by direct observation. They have been seen and photographed jutting from the limb as rotation brought them into, or carried them out of view. It is also inferred from their superior brilliancy, which might serve to measure the opacity of the veil spread over the surface they surmount. On the other hand, they escape none of the Fraunhofer absorption; the whole range of solar dark lines is invariably present in their analysed light. Hence their position can be defined as intermediate between the “smoke veil” and the reversing layer. This consideration affords a safe holding-ground for reasonings about the status of these remarkable objects. They testify to internal commotions of the same nature as those giving rise to spots, but exempt from their heliographic limitations. Faculæ are not confined to the spot-zones; they develop all over the solar globe. Since, however, spots invariably claim their attendance, they are most numerous in the latitudes frequented by such disturbances, while showing independent maxima much nearer to the poles.[169] But their imperfect visibility upon the disc greatly restricted their observation, until Professor Hale and M. Deslandres almost simultaneously invented a method for spectrographically recording them.[170]

It depends essentially upon the use of a double slit—Janssen’s valuable invention for isolating spectral rays. Celestial objects can in this way be photographed in monochromatic light—that is to say, their forms in each separate chemical element can be distinctively recorded. The importance of the fresh start thus made is difficult to exaggerate. With a double slit and a sensitive plate, the comparative distribution of glowing vapours in the sun can be satisfactorily investigated, and anomalies connected with their distribution, if not removed, at least fully recognised and defined. All spectral rays, nevertheless, are not equally available for all purposes. In chromospheric photography, for instance, the calcium H and K offer immense advantages, not only because of their actinic efficiency, but still more on account of the broad bands of shadow rendering them conspicuous as Fraunhofer lines. These serve to protect against atmospheric glare the bright lines superposed upon them at the edge of the sun; and glare is the worst foe of daylight photography. In systematic work, moreover, K is, for more than one reason, always preferred to H. And so came to be established, in two continents at once, a new branch of astronomical art—the art of picturing the sun and its surroundings in calcium-light of a single quality.

The first and immediate object in view was the day-by-day photography of prominences; but it was very soon found practicable to extend the work from the limb to the disc. One slit was caused to travel across the sun’s image, which had a diameter of two inches in the Chicago twelve-inch refractor, while the motion of the second was adjusted so that it exactly kept pace with the K-line, admitting it alone, through a chink just two thousandths of an inch in width, to impress the sensitive plate. At Chicago, the first experiment of the kind was made 28th December 1891, and a similar mode of procedure was described by M. Deslandres before the Paris Academy of Sciences, 8th February 1892.[171] The upshot in each case was the discovery that K is doubly reversed over extensive tracts of the photosphere. Plate IV., Fig. 1, shows the sun self-portrayed in calcium light, 11th April 1894. The lines crossing the photograph are accidental imperfections, one set originating from dust-particles in the jaws of the slit, the other from irregularities in the movement of the siderostat. The regions of reversal are not confined to the neighbourhood of spots, but spread irregularly over the sun’s surface. The individual bright forms they include are often bent into spirals or doubly curved. They correspond very closely, both in aspect and position, with faculæ directly seen, and were at once, by Professor Hale, identified with them. M. Deslandres, however, classing them as a species of hybrid between faculæ and prominences, bestowed upon them the compound name, expressive of this mixed quality, of “facular flames.”[172] Hale’s view, in other words, was that the novelty disclosed by his “spectroheliograph” consisted in the emission by faculæ of bright “H and K” by way of supplement to their regular photospheric spectrum, while Deslandres considered that the new investigation applied to a distinct kind of objects, neither prominences nor faculæ, although partaking of the nature of both. The question raised is difficult; let us briefly examine its bearings.

The calcium-flames photographed at Paris and Chicago are certainly not prominences. Their positions do not coincide with those of the chromospheric offshoots. They appear where prominences are not, and _vice versâ_. Nor are prominences bright enough—unless by a rare exception—to show in relief against the sun; they are strictly objects for marginal spectroscopic study. Finally, motion-displacements are conspicuous in them, and are absent from the sinuous shapes on the disc. The objections to identifying these with faculæ are less obvious, yet seem equally insuperable.

PLATE IV.

1. The Sun portrayed in Calcium Light. From a Photograph by M.
Deslandres.

2. Photograph of the Chromosphere and Prominences taken by M.
Deslandres at 5 h. 40 m. P.M., 31st May 1894.
]

Faculæ belong no less unmistakably to the photosphere than the Himalayas do to the earth’s crust. They are flung upward from it; they subside back into it. Their light is of the same brilliantly continuous quality; it is marked by the same kind and amount of linear absorption. It has then been sifted through all the vaporous strata amassed above the surface of the sun. This gives the key to the position; for it compels the inference that while faculæ are situated beneath the reversing layer, the allied gaseous forms must be located above it. The argument for their comparative elevation is cogent. The reversing layer, as we know, is rich in calcium vapour cooler than the photosphere, and, _à fortiori_, cooler than the flames detached by their superior lustre from the photosphere. Their calcium-rays should, accordingly, be effectually stopped by the reversing layer; they could not by any possibility be transmitted through it. Hence the bright “K” in which the solar disc can be depicted must originate above the region where Fraunhofer absorption takes place. The wreathing forms emitting it belong to a different locality from that of true faculæ. They are chromospheric, not photospheric, developments. Yet, as just stated, they cannot be assimilated to prominences. They probably lie at the base of the chromosphere, and they follow the distribution of faculæ with close, though not rigid exactitude. Hydrogen is at times represented in their spectra by a crimson glimmer of C, so they are not composed of calcium solely; but their real nature and place in the sun’s economy remain admittedly enigmatical. We are only sure that they spring up in close relation with faculæ as symptoms of the same disturbances. That their relation to them is one of actual identity seemed indeed at first sight scarcely open to doubt; yet for the reasons just assigned they must be regarded as separate phenomena.

The connection of faculæ with spots is not, as we have seen, of the “mutual” kind. Their reciprocity—as a typical Irishman might say—is one-sided. Outside the spot-zones, where solar commotions have a less vehement character than nearer to the equator, faculæ lie “extended many a rood” in sluggish inactivity during intervals long enough for the working out of manifold changes in the brilliant circumvallations of spots. These are very often crowned with prominences, which, however, develop somewhat tardily. Nascent umbræ are rarely accompanied by them, while moribund umbræ offer a favoured site for their growth. Thus a remarkably brilliant prominence, agitated by violent motion, towered above the eastern limb as it was passed, on 3rd March 1892, by the shrunken remnant of the vast spot-group which had slipped out of sight round the western limb a fortnight previously.[173]

Prominences are related to the chromosphere in very much the same way that faculæ are related to the photosphere. They arise from it by effluence or eruption. Now the chromosphere itself has a markedly eruptive aspect. It presents no billowy ocean surface, but resembles rather a Tartarean meadow planted with stalks and grass blades of fire, waving under some unimaginable furnace-blast. With clear definition, a filiform texture is everywhere apparent. The “straw-thatch” effect of penumbræ denotes a similar peculiarity in photospheric materials, and that it is shared by prominences can be inferred from their frequent construction, as if out of the untwisted strands of a rope.[174] The red rim of the eclipsed sun is conspicuously jagged, and its saw-like outline suggested the name _Sierra_, originally bestowed upon it by Airy in 1851. It does not, then, represent a fluid envelope in a state of equilibrium. There is another reason why this is impossible. The chromosphere does not preserve a uniform depth. Averaging about 5000 miles, it is subject to irregular and temporary variations, as yet imperfectly observed and entirely unexplained. General subsidences are even affirmed to take place. One such was noted with surprise by Trouvelot, June to August 1875.[175] He had frequently observed extensive local depressions, but never before a universal shoaling. He attributed to the sharper views of the photosphere afforded by this removal of part of the interposed medium, his discovery of “veiled spots,” imperfect umbral formations, owing their abortive character, perhaps, to their unfavourable situation near the poles. In 1887, the height of the chromosphere, measured thirty-two times by Dr. Fényi, S.J.,[176] was found to diminish above the spot-zones, and sensibly to increase outside their limits. It would be of interest to determine whether this condition prevails commonly, or whether it is restricted to epochs of minimum.

Prominences are of two varieties—eruptive and quiescent. These differ chemically, visually, and heliographically. Eruptive prominences are of a more mixed constitution than the quiescent sort; they include many more ingredients; they give an intense carmine light, have jet-like or upspringing shapes, and are mostly confined to the spot-zones. They are genuine fire-fountains, while quiescent prominences frequently resemble cloud-banks. The former are individual outbreaks; the latter unite into communities, counterfeiting banyan groves, ranges of jungle, fields of cirrus. Usually connected with the chromosphere by stems or trunks, they in some cases, not only float free in isolated masses, but, still more remarkably, are generated at great elevations above it, as if by the spontaneous illumination of pre-existing material. The analogy with terrestrial condensations in an azure sky is obvious, but may be quite misleading.

Quiescent prominences develop on an enormous scale, nor always tranquilly. Formations, at least, which are in some respects cognate with them, become, on occasions, the scenes of striking explosive accidents. So that their generic title must be understood in a restricted sense. Thus the cloud-like character of a huge object photographed by Deslandres,[177] 31st May 1894, seemed vouched for by its vicinity to the south pole, and by the large extent of the solar limb garnished by it. At 2 P.M., when it was first pictured—as usual, in calcium light—its height, apart from foreshortening, was 2′ 20″, or 63,000 English miles. At 4^h 27^m it had sprung up to 135,000, and 73^m later still to 270,000 miles (see Plate IV., Fig. 2), an elevation far beyond any recorded for objects of the kind situated more than sixty-five degrees from the solar equator, while this wonderful structure spread from the seventieth to the eighty-first southern parallel. Perceptibly filamentous, it seemed to grow by the elongation of its component threads or ribands, and included within its vast bulk probably the minimum conceivable quantity of matter. On the same plates, a group of smaller but intensely active prominences registered themselves at the solar antipodes.

FIG. 12.—Prominence observed at the Haynald Observatory, 19th Sept.
1893.

The spectroscopic line-displacements due to motion are illustrated in
the upper sections of the figure.
]

An example of a transient apparition, difficult to classify, is given in Fig. 12. It simulates a portentous conflagration. From bottom to summit the red pillar of hydrogen measured 166,000 miles, according to Dr. Fényi’s observation at 2^h 20^m, 19th September 1893. But it wholly lacked any interior principle of stability. Opposite, and excessively swift movements in line of sight of the base and shaft betrayed the progress of destructive change. The catastrophe was not long delayed. Within less than half an hour all was over. A rush upward set in at the rate of 132 miles a second, carrying the frail edifice to a height of 224,000 miles. A minute and a half later, it had faded and dissolved, “like the baseless fabric of a vision,” leaving just “a rack behind” in the shape of an insignificant protuberance hedged in with faculæ. “Throughout the course of its appearance,” Dr. Fényi wrote,[178] “the entire object consisted simply of very bright luminous bands or strips scattered one after another in ragged forms, and apparently lying nearly at right angles to the limb of the sun. They were strikingly bright even in the highest parts of the prominence. The form as a whole was also like a band or stripe, which had no pronounced inclination, but stood erect nearly in the direction of the sun’s radius.”

This amazing outburst was repeated with enhancements, nineteen hours later, at a point on the sun’s limb almost diametrically opposite to its predecessor. The height in this case attained was 300,000 miles, the mean velocity of ascent being 214 miles a second, besides which, retreat from the earth was indicated for the entire mass at the rate of close upon 160 miles a second. A convulsion at least equally violent was witnessed by the same observer on 24th December 1894,[179] when a brilliant and wide-spreading, yet tolerably tranquil prominence, 56,000 miles high, suddenly began to mount, and attained in thirty-five minutes the towering stature of 300,000 miles. Speedy disorganisation ensued. Less than two hours after the explosion its scene was vacant. The body affected by it had been shattered out of existence by its destructive violence.

The Kalocsa observations were made with a visual spectroscope. Simultaneous photographs would have been of especial value for comparison with them, but leisure was not afforded for combining methods. The three objects they referred to were palpably identical in character. All showed precisely the same kind of structure. They were made up of glowing vertical bands, collected into loose sheaves, or scattered in outlying detachments. Each enormous aggregation, too, stood erect during the tumultuous processes of expansion and collapse. They were related to spot-groups, if at all, only by _diametrical opposition_. The singular counterbalancing tendency of solar disturbances is frequently conspicuous, and perhaps rarely or never absent. Two gigantic, although ephemeral apparitions, thus coupled, were, for instance, observed by Trouvelot, 26th June 1885;[180] and on 16th August of the same year he noted the apparent connection of a prodigious chromospheric outburst with a spotted condition of the limb 180° distant. A relief of pressure may be concerned in these phenomena, as in volcanic eruptions; if so, the lift, or diminution of gravity, must act right across the solar globe, quickening convection-currents and facilitating the antipodal delivery of extra supplies of heat. In point of fact, the usual premonitory symptom of the explosive development of prominences is a disengagement of light.[181] Exceptional brilliancy is the forerunner of abnormal activity. No satisfactory cause, however, can be assigned for the imagined relief of pressure, since tidal influences are fairly out of the question. Only one undeniable inference can be derived from the contrary symmetry of solar commotions. It is that they are extremely deep-seated. They have their roots in the hidden profundities of the great globe they agitate exteriorly. They are not then mere local accidents; they make an intimate part of the solar economy. The recognition of this characteristic is interesting and important.

The tallest prominences are ordinarily the least coherent in structure. An example is shown in Fig. 13, from a drawing made by M. Fényi, 3rd October 1892. The object it portrays was almost unique in its fantastic and colossal form. Overarching thirty degrees of the limb, it rose above it to a height of nearly a quarter of a million of miles. Its fragmentary composition was patent.[182] Under the eyes of the delineating artist it was “in the act of being blown to shreds.” Photographs happily secured at Chicago seven hours later, exhibited it as then fallen to less than one-quarter of its high estate, while of augmented lateral spread. These represented, of course, its calcium aspect, the Kalocsa drawing its shape in red hydrogen; but there was no evidence of the two vapours being differently distributed throughout this bubble edifice. Facular patches marked, on the sensitive plates, the points both of rise and re-descent of the materials constituting it. Next morning only some insignificant wreckage strewed its place; virtual annihilation had overtaken it.

FIG. 13.—Prominence observed at the Haynald Observatory, 3rd October
1892. Height, 8′ 51″.
]

The tremendous velocities observed in prominences constitute a formidable problem in solar physics. They not infrequently exceed the critical rate of 383 miles per second; that is to say, they transcend the limit of the sun’s gravitational controlling power, and, apart from possible resistance by a medium, should carry the substances animated by them finally away into space. The shapes and motions of prominences, however, clearly indicate some retardative action,[183] although its excessive feebleness, at least in coronal regions, can be inferred from the unimpeded circulation of comets passing within a hundred thousand miles of the sun’s surface. Hence it is only a surmise, not a certainty, that chromospheric outbursts are attended by irrevocable loss of matter. The swiftest so far recorded took place, 17th June 1891, in connection with a spot just disappearing through rotation. A concentration of vivid luminosity gave the signal that something unusual was impending, “and after six P.M., Kalocsa mean time,” M. Fényi wrote,[184] “the point at 281° shone with so great a brilliancy that its reddish light seemed to become white; an enormous displacement of the spectrum towards the blue, at a medium height above the sun’s limb, indicated at the same time an approach of the hydrogen in our direction with a prodigious velocity.” The entire object, which was composed of glowing filaments, lay, in fact, on the more refrangible side of the C-line, as the result of approaching speed up to 550 miles a second. A vertical ascent at the rate of 300 miles being meanwhile directly visible, a total velocity (neglecting an uncertain third component) of about 680 miles a second must have been attained in this amazing explosion. Trouvelot’s observation a few hours earlier of a brilliant and peculiar facular blaze at the place where it occurred[185] was significant of the intense energy pent up in the spot-group, waiting release by the figurative trigger-touch.

This prominence easily wins the prize for rapidity of movement; yet the competition has been keen. Velocities of the same order, arising under similar circumstances, have been frequently determined. They appear, indeed, improbable, and have been stigmatised as “fabulous.” No actual transport of matter, it is alleged, can be in question, but merely a swift transference, through gases previously obscure, of a luminous condition.[186] But this does not account for the conspicuous shiftings of spectral lines, which could not ensue from the progress of incandescence through stationary matter. They demand the strict application of Doppler’s principle; real velocities must correspond to them. Nor, even if there could be deception about the movements of prominences _in_ the line of sight, is illusion possible about those _across_ it. And both kinds are of the same order of speed; they are complementary; they represent different aspects of identical disturbances. This is not all. Prominences are often visibly twisted; they are composed of spirally mounting flames; while to this helical conformation correspond gyratory movements, at times disclosed by the spectroscope. Here, at any rate, we have to do with bodily transportations of matter; “luminescence” cannot be propagated vortically. Besides, the measured speeds are as difficult to explain on one hypothesis as on the other. Chemical action does not spread instantaneously. Through the tenuous gases of the chromosphere, a maximum rate of one mile a second might be assigned to its progress—a rate, that is to say, some hundreds of times slower than the velocities to be explained. That they are somehow of electrical production is a safe assertion, likely to be true, if not in an immediate, then in a remote sense. Yet we are little the wiser for the admission. The “floating of an idea” in the mind does not constitute knowledge; and a speculation is only valuable when it offers a definite starting-point for practical research.

Total eclipses have ceased to be indispensable for the prosecution of chromospheric studies. Day by day the red rim of the sun, with the strange forms protruding from it, can be viewed spectroscopically; and day by day the same objects vestured in violet can be photographed under the broad shelter of the Fraunhofer K-line. Nevertheless, noontide darkness, when it comes, brings very appreciable help. Differences are noticeable between what can be seen in and out of eclipse. According to the late Professor Tacchini,[187] the chromosphere always appears deeper under cover of the interposing moon, because it is surmounted by a pink-white margin, giving continuous light, and therefore spectroscopically invisible in daylight. Some prominences are probably of analogous composition. Only their skeleton-forms come out in the crimson radiance of hydrogen; they are compacted and clothed with white materials, the shining of which is effaced by the glare of common day. A spectroscopic survey of the chromosphere and its appendages should hence always be made immediately before and after every eclipse, for comparison with the direct photographic records obtained during the corresponding totalities. For the present the information acquired by daylight work at the edge of the sun remains under a partial slur of incompleteness.

The objects called “white prominences” belong indeed wholly to the pageantry of eclipses. First noticed by Tacchini at Caroline Island, 6th May 1883, they showed as lucid jets about a hundred thousand miles high, with a surface like granulated silver. Attempts made, after the return of daylight, to view them prismatically proved fruitless; they gave forth no hydrogen or helium rays. Again at Grenada, 29th August 1886, a gigantic helical structure, described by Mr. Maunder as “of the intensest silver whiteness,”[188] towered three hundred thousand miles above the limb of the moon. Its spectrum, photographed by W. H. Pickering, included bright H and K, and Professor Hale accordingly entertained the hope that such objects might come within the range of his spectroheliograph; but so far no trace of them has been caught outside total eclipses.

Dark chromospheric forms are not unknown. A “black protuberance,” observed by Trouvelot at Meudon, 7th October 1892,[189] might have been only a negative impression, like “black” flashes of lightning; yet it in a measure falls into line with eclipse-records certainly not due to illusion. In the great “anvil protuberance” disclosed 7th August 1869, Dr. Lewis Swift saw “many black lines crossing in different directions, and inasmuch,” he added, “as they must have been, at least, fifty thousand miles long and a thousand miles broad, it would appear to be important to understand the cause of this phenomenon, and (to ascertain) if these markings are always present.”[190] Corroborative observations were made by Alvan G. Clark and Professor George Davidson. With them may be usefully compared M. Liais’s description of a black-edged but colourless prominence watched at Paranagua, in Brazil, during the eclipse of 7th September 1858.[191] The composite effect was not due to contrast, since the obscure summit stood out alone when the bright lower portion had disappeared behind the advancing moon. These singular phenomena excited little comment, and lapsed into oblivion, so that dusky ramifications, connecting and defining prominences in the eclipse-photographs of 9th August 1896,[192] seemed entirely novel features. They were explained by M. Hansky, a member of the Russian party on the Amur, as outflows of hydrogen cooled by expansion;[193] but inadequately. Hydrogen below the temperature of luminosity is transparent, and the photographed veinings were densely opaque. They afford a hint, that will certainly not be disregarded, of the workings of unknown activities in connection with prominence-development.