Chapter VIII: The Spectrum of Sun-Spots

Sun-spots give a remarkably compounded spectrum. It appears to sum up five different sets of effects. That is to say, the obscure longitudinal stripe corresponding to the umbra owns a quintuple origin. Each of its elements might be made the subject of a somewhat prolonged discussion. For the sake of clearness we will briefly enumerate them. They consist of: (1) A bright background of ordinary photospheric light; (2) a nearly continuous band of dense absorption, extending from the infra-red to the ultra-violet; (3) a select array of widened Fraunhofer lines; (4) nearly all the Fraunhofer-lines under a normal aspect; (5) a restricted number of bright lines.

Dunér of Upsala ascertained in 1891[151] that the _fundamental_ radiance of spots is indistinguishable from that of the general surface of the sun. Their darkness is then due to increased absorption, not to diminished radiation. This fact decisively negatives some current theories, and thus limits the field of speculation as to the nature of spots. They are shown by it unmistakably to be regions where cooled materials of some kind accumulate. Of what kind those materials are, we can learn something—although not by any means all that could be wished—from their peculiar modes of arresting light.

A section of a spot-spectrum in the yellow-green is portrayed in Fig. 10 from a photograph by Professor Young. The belt of strong absorption which is its leading feature seems, but is not really, continuous. Young himself discovered in 1883 that it is made up of innumerable fine dark lines, set very closely together, or even actually overlapping. Each individual in the multitude is, he tells us, “spindle-shaped—_i.e._ thicker in the middle where the spectrum is darkest—and tapers to a fine hair-like mark at each end; most of them can be traced across the penumbra-spectrum, and even out upon the general surface of the sun.”[152] These observations, which require a high resolving power in the apparatus employed, were amply confirmed by Dunér. He perceived further that the lines are collected into groups, leaving chinks of undimmed photospheric brilliancy between.[153] Within the limits of the “_b_-group” alone, no less than 300 of these dusky _fibres_ were counted; above F, however, they become merged together by crowding, and below E by diffusion. Their separation and arrangement are most evident in quiescent round spots with intensely black nuclei—in spots, that is to say, commonly regarded as of the minimum type. Now such a spectrum as they constitute cannot be produced by liquid or solid matter, however minutely subdivided; it decisively claims a gaseous origin. Hence the darkening in spots is not merely an intensification of the “smoky” absorption veiling the entire disc; it is special and peculiar. So much can be safely asserted.

FIG. 10.—Portion of Sun-spot Spectrum, photographed by Professor Young
in 1893.
(From Young’s _General Astronomy_. By permission of Messrs. Ginn and
Co.)
]

Perhaps the most distinctive part of the spot-spectrum is the collection, included in it, of accentuated Fraunhofer lines.[154] They are picked out to be widened and darkened on some recondite principle of selection, which varies from spot to spot, and from epoch to epoch. This was early noticed by Sir Norman Lockyer, and he pursued the inquiry with striking results. The discussion in 1886 of observations upon the spectra of seven hundred sun-spots, made at South Kensington on a fixed plan during six years, led him to the following conclusions:—[155]

(1) “The most widened lines in sun-spots change with the sun-spot period.”

(2) “At, and slightly after the minimum, the lines are chiefly known lines of the various metals.”

(3) “At, and slightly after the maximum, the lines are chiefly of unknown origin.”

In other words,[156] “As we pass from minimum to maximum, the lines of the chemical elements gradually disappear from among those widened, their places being taken by lines of which we have at present no terrestrial representatives.” “Dissociation,” in short, was the _mot de l’énigme_. As the sun’s temperature increased with the growth of disturbance, substances in a terrestrial sense “elementary” were supposed to split up into exotic constituents, giving spectral lines strange to laboratory experience.

The evidence for the progressive change thus interpreted was indeed slight, except as regarded iron; and iron alone was taken account of in the confirmatory Stonyhurst observations. So far as they went, however, they were decisive, and all the more so that they covered a different spectral range (B to D) from that (D to F) examined at South Kensington. They showed demonstratively that, throughout the disturbed interval between January 1884 and October 1886, iron lines were all but completely replaced by “unknown lines” in the list of those affected in spots, while they duly reappeared upon the restoration of photospheric tranquillity. In connection with their behaviour, nevertheless, Father Cortie established an important distinction. Their presence or absence he found to be determined, not by the general flow of solar commotion, but by the nature of individual spots. In those of rent and ragged aspect and tumultuous proclivities, iron lines are ousted by unidentified faint rays; but in tranquil spots the iron spectrum is at all times prominent. And since the former sort prevail at maximum, the latter at minimum, the statistical outcome is that the spectral variations appear to depend simply upon the great cyclical pulsation of the solar globe. Only on special examination they prove to be determined more locally and particularly than this would imply. In some unquiet spots, for instance, which developed near the minimum of 1889, the effacement of iron lines was as complete as if the epoch had been one of maximum. In fact (as the Stonyhurst astronomer remarked),[157] the widening of unknown lines is common to all stages of solar activity, provided spots of an appropriate character be at hand. This is not a distinction without a difference. It cuts the ground from under the assumption of periodical vicissitudes in the general chemistry of the sun. Iron is not everywhere, and inevitably reduced there to its elements as temperature and disturbance culminate together, but—if at all—only as a special effect in the hottest spot-craters. And this again brings up difficulties connected with relative temperature—difficulties which, in one form or another, perpetually recur in the study of astrophysics.

But there is more to be said. Further inquiries have materially altered the aspect of the case, for they have led to the transference from the “unknown” to the “known” class of so many spot-lines that the completion of the process may be confidently anticipated. Rowland’s photographic comparisons have contributed most effectually to its advance. Young and Cortie have traced a crowd of sun-spot rays to vanadium; titanium claims as many, or more; and others perhaps originate from allied “rare” metals. This singular line of identification is very strongly traced. Thus _all_ the vanadium lines, twenty-eight in number, between C and D are by turns broadened in spot spectra, although of evanescent faintness in the photosphere; nor does the conjecture seem unwarranted that the high temperature compounds with nitrogen and oxygen, both of this metal and titanium, may yet be recognised in umbral chemistry. The distension in a spot of two vanadium lines, at λ 5728 and λ 5731 respectively, is well shown in Fig. 10. They are of quite minor importance in the Fraunhofer spectrum.

Father Cortie surmises that the vapours absorbing in spots may be associated by their approximate conformity to a certain standard of density. “The level of sun-spots,” he suggests, “is possibly the level of the faint lines of such metals as have an atomic weight about 50.”[158] Iron, nickel, titanium, and vanadium, all assiduous frequenters of umbral cavities, belong to this category. But the rule is compromised by exceptions and incongruities.

The actual state of the case is this. There is no evidence of elemental dissociation in sun-spots, but spectral diversities are obvious and persistent. They indicate the disappearance of iron from tumultuous formations, and the emergence in them of titanium and vanadium. There are doubtless concomitant changes, but they await ascertainment and particularisation. A _caveat_, however, has to be entered. The principle upon which these inquiries have been conducted is imperfectly assured. It is commonly taken for granted that the widened lines constitute the spot-spectra; that they, and they alone, represent the emanations of the constipated vapours blotching the lustrous disc. But this is a somewhat arbitrary assumption. The theory of line-expansion by pressure is very imperfectly understood. The phenomenon does not occur uniformly and invariably. Lines of different substances are differently affected by it; lines even of the same substance are unlike in their susceptibility to its influence. The inferential building up then, of spot-spectra out of widened lines is subject to many qualifications. These do not lessen the importance of the observed relation, but they importantly modify it.

The iron lines intensified in spots, presumably by the specific action of their nuclear vapours, are often unsymmetrically broadened. They are usually diffuse towards the violet side, sharp to the red. This may point to the presence of chemical compounds;[159] since the flame-spectra of metals and of their oxides seem to be differentiated just by the development, in the latter, of these peculiar shadings. The possibility must accordingly be admitted that iron-oxides exist in the sun. Yet the implied temperature is improbably low, since they can be broken up here on the earth by the simplest metallurgical processes. But they might perhaps form transiently (so to speak) in spots, as a result of the local chilling of swiftly circulating material. It may be added that the iron lines distinctive of spot-spectra are so-called “low-temperature lines.” They are brilliant in the electric arc, but tend to be outshone by others in the higher excitement of the spark.

Further complexity was imparted to spot-absorption by the appearance of certain dusky bands, of which nine, situated below D, were observed early in 1885 at Stonyhurst,[160] and no fewer than seventeen more refrangible in 1880–3 at Greenwich. One proved identical with a fluting drawn by Young in 1872,[161] and all were resolvable into densely packed lines. Nothing is known, or can even be conjectured, as to their origin; but they are clearly symptoms of disturbance, since, with the sudden advent of a solar calm in October 1886, they at once utterly vanished.

Mr. Evershed considers the majority of the unaffected Fraunhofer lines in spot-spectra to be possibly spurious;[162] they may, he thinks, be inherent, not in umbral light, but in the photospheric glare diffused equally over spots and the surrounding sky. Some of the ordinary dark solar lines, however, thin out in crossing umbræ, and a few show traces of partial brightening. Moreover, the radiations from spots cannot escape transmission through the reversing layer, and are hence subject to precisely the same absorption exerted upon sunlight in general, so that the Fraunhofer spectrum in its integrity truly belongs to spots, notwithstanding the reinforcement of some of its components and the enfeeblement of others through influences special to them.

One of the most remarkable features of spot-absorption has still to be noticed. Helium, as we have seen, makes no show in the Fraunhofer spectrum. Yet a helium-envelope surrounds the sun to a depth of five thousand miles. Every ray of sunshine sent abroad into space has been sifted through this huge volume of gas, which, by its anomalous inertness, bids defiance to “Kirchhoff’s law.” Emitting complex ranges of vibrations, it nevertheless exacts no corresponding toll of absorption. Its transparency seems absolute. Either it is so hot that it replaces the light arrested, or its arresting power is nullified by rarefaction. The former alternative is excluded by the consideration that an excess of temperature should be notified by the presence of bright helium lines in the general spectrum of the sun, and they are no more visible in it than dark ones. Hence the absorptive incapacity of chromospheric helium may provisionally be attributed to extreme attenuation.[163] This view has gained plausibility through the discovery that helium in or near spots acts at times normally upon light, for the vapours and gases producing umbral obscurity are assuredly, on any theory of spot-formation, denser within than outside the apparent cavity. The shading then at D_{3}, like the fusiform shape of the sodium pair below it (see Fig. 11), results from increased pressure. It is usually significant of vehement disturbance. In twenty or thirty spots with flaming appendages—and mostly in their penumbral regions—Professor Young has seen the yellow helium ray reversed;[164] it was similarly visible to Professor Naegamvala in the huge vortex of February 1892,[165] and to Mr. A. A. Buss of Manchester, on 17th March 1899,[166] in a spot the incessant activity of which was the more remarkable on account of its occurrence near an imminent minimum. Absorption by the deep red helium ray at λ 6678 was three times observed in spot-spectra by Father Perry during 1883,[167] but its chemical meaning was then unsuspected. Now that the helium-spectrum has been unravelled, further particulars might easily be learned as to the associates of D_{3} in spots. Its isolated occurrence is improbable.

FIG. 11.— Reversal of the D-Lines in the Spectrum of a Sun-spot
(Young).
]

Spot-spectra are crowned and completed by the frequent superposition upon them of vivid rays. These originate from the gaseous effusions often accompanying the formation and transformation of spots. They are readily identifiable. Hydrogen lines and the H and K of calcium are the most frequently brightened; D_{3} is sometimes bright over the umbra, dark in the penumbra, of the same spot; and “double reversals” of the sodium “D pair” are quite commonly observed. The phenomenon is illustrated in Fig. 11. The brilliant ray shining at the core of the fuzzy spindle, into which each of the coupled lines is broadened, evidently proceeds from an overlying hotter and rarer stratum of sodium-vapour. The magnesium group “_b_” is occasionally affected in the same way. The “rosy veils” in spot umbræ give out, as might be expected, hydrogen rays, and “bridges” are also loci of emission. The ultra violet members of the hydrogen series are never present, bright or dark, in spots; and the fifth line (Hε), which falls just within the region of visibility, has often been looked for in vain. In some of Professor Hale’s spectrographs, however, of the giant spot of February 1892 it showed faintly bright beside the more conspicuous H of calcium.[168] So far the record stands alone. It has a particular interest from the ambiguous position occupied by this ray in the solar spectrum.

The agitation prevailing in spots is often betrayed by line-distortions, telling of the swift recession or approach of vapours congregated in them. The condition of the C-line, as sketched by Professor Hale, 13th February 1892, in the same spot, is shown in Plate VI., Fig. 2. The brilliant patch over the umbra is of normal wave-length; it was derived from a flame radially immobile; but the hook-like appurtenance of the dark line testifies to an extraordinary outrush of cooler gas from the lower part of the formation. We can, to a certain extent, trace its course. It started outward with a uniform velocity away from the earth of about 120 miles a second. This slackened unequally, as can be seen by the breadth of the “hook” at its junction with the line; and the whole mass of hydrogen came to rest at a distance of thirty to forty thousand miles from the point of issue, which (it is worth noting) was at the very middle of the nucleus. The nature of the force raising this brief but tremendous storm cannot readily be imagined. Its abrupt development marked an acute crisis of disturbance, to which the earth responded with magnetic twitches and auroral illuminations.

Motions in spots seem to be limited and local. They can be inferred to prevail with great violence at certain levels, while complete tranquillity reigns at others. This, at least, is the only explanation of the chemical peculiarities of solar hurricanes. Nothing, for instance, is commoner than the raging of hydrogen-storms amid profound calcium-calms. Nay, lines belonging to the same substance may indicate for it simultaneously rest and motion. Thus a few iron lines are at times observed to be displaced or twisted through the effects of rapid approach or recession, while the remainder maintain their usual positions and aspect. The anomaly is most striking, and challenges persistent attention. Sir Norman Lockyer meets it with the dissociation-hypothesis; but this raises more difficulties than it removes.

Enough has been said to show that numerous and most curious problems await solution by students of sun-spot spectra. The subject is wide enough to occupy a band of specialists, and its remoter implications can still be only surmised. Nevertheless, definite conclusions are not wholly out of reach. First, as to the cause of nuclear darkness. It is certainly to be found in augmented, and (so to speak) reiterated absorption. Spots are not simply rents in a shining veil, exposing an obscure substratum. They are not super-heated regions, where processes of condensation are suspended. The photosphere is screened, not perforated, by them. Moreover, the screening is by interposed vapours. Umbral absorption is mainly, if not altogether, of the gaseous kind. It is essentially linear and banded. No part of it can be safely attributed to the action of a foggy precipitate such as modifies elsewhere the “surpassing glory” of the disc. They probably differ in this respect from “pores” and “veiled spots,” but specific inquiries on the point have yet to be made.

There are strong indications that spot-spectra originate under conditions of increased pressure and diminished temperature. Still the coolest umbræ must be hotter than the reversing layer, for otherwise the Fraunhofer lines would show bright against them, and, as we know, they cross them in dusky array. This circumstance is fundamental in solar thermal relations, yet has been generally overlooked. The ordering aright of such relations is a prime desideratum in solar physics, and should serve as an indispensable guide to the interpretation of spectral diversities.

PLATE VI.

1. The Corona of 1900. Drawn from Photographs by L. E. Jewell.

2. Reversal and Distortion of the C-Line in Sun-spot (Hale).
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