Chapter X: The Chromospheric Spectrum

The spectrum of the chromosphere is almost purely discontinuous. It is composed of detached bright lines. Some of these are always present, but the majority come and go. We will consider first the permanent radiations.

The chief of these are readily identified; they belong to hydrogen, helium, and calcium. Thirty hydrogen lines have been seen or photographed in dispersed chromospheric light, all members of the original, or “Huggins series,” which attained nearly to its theoretical limit on plates exposed by Mr. Evershed during the Indian eclipse of 1898. The fundamental C (Hα) is the brightest. To its intensity is due the crimson glow of chromosphere and prominences; and prominences viewed spectroscopically on this line appear larger than when imaged in any of the other qualities of hydrogen-light. This is generally explained by the influence of temperature, a higher degree of heat being needed to give complete development to forms of shorter wave-length.[194] A comparatively moderate chromospheric temperature would thus be indicated. But there may be other influences in question. Professor J. J. Thomson made the significant observation in 1895[195] that the red and green lines of hydrogen show marked differences of intensity at the two electrodes of vacuum-tubes, the red predominating on the positive, the green on the negative side of a partition. The hint must indeed be reserved for future use. We are not yet in a position to apply it profitably.

The helium-spectrum of the chromosphere also gives rise to some interesting considerations. For its various constituent series are not represented indiscriminately at the edge of the sun. These series, we may remind our readers, are six in number, and they are distributed with beautiful precision into two corresponding triple systems. They may be distinguished for convenience as “yellow” and “green,” D_{3} giving the _tone_ to the former, several vivid green lines characterising the latter set. In the laboratory, as we have seen, they are inseparable; one set cannot, by any artifice so far devised, be procured apart from the companion set. But it is otherwise in the sun. Yellow helium is always present in the chromosphere and prominences; green helium only about one-fourth as often as it is looked for. The permanent chromospheric lines are four, namely, one far down in the red at λ 7065, the familiar D_{3}, a deep blue ray at λ 4472 (formerly known as “_f_”), and the ultra-violet “leader line” of the principal series at λ 3889. These have probably many associates of still shorter wave-lengths; but photographic data are too scanty as yet for purposes of discrimination between the constant and the occasional elements of the spectrum. They do not proceed, it must be borne clearly in mind, from two substances, but from one indivisible form of matter, differently conditioned. In what way, it is not easy to imagine. Laboratory experiments show that the green set of lines gains relatively in brightness with rarefaction, although the yellow set persists as well to the limit of practicable exhaustion. This indication, however, does not open a way out of the chromospheric difficulty. The gases near the sun are of inordinate subtlety. Helium ought there, if this alone were the determining quality, to be in the _green_ state. Its most fundamental emanation, nevertheless, is D_{3}. Nor is its dominant position compromised in the highest prominences. On the contrary, the green rays nearly always proceed from lower lying, and therefore from denser strata than the yellow.[196] Supplementary influences are then active—temperature, mode of illumination, admixture of foreign materials. To this latter cause of spectral modification, helium, we know, is abnormally sensitive.[197] And it is quite possible that some of the series emitted by it may be more liable to suppression than others, in which case the emergence of the yellow without the green set would be an effect of damping, not of density. It must, nevertheless, be admitted that what little relevant experimental evidence there is, scarcely countenances this surmise. The complex spectra derived by Professors Liveing and Dewar[198] from the volatile residuum which survived the freezing-out of the main constituents of atmospheric air, included rays taken impartially from all the helium-series. No quantitative analysis of the contents of their tubes was, however, possible; and the fact has been otherwise learned that the helium-ingredient of a blend must be predominant to become spectroscopically conspicuous. Every volume of hydrogen present in the chromosphere (neglecting the effect of metallic vapours) should thus probably be diluted with two volumes of helium; constituting the solar appendage largely a helium envelope. This important piece of information was brought within reach only by terrestrial observation of the new element captured from clevite.

The significance of calcium in its chromospheric relations has only of late been fully recognised. And this for an obvious reason. It is represented by only one pair of lines—bright H and K—and these are so near the limit of visibility that they could be effectively studied only by the aid of photography. They were indeed registered as leading features of the chromospheric spectrum by Professor Young in 1872; but he was entirely incredulous as to their calcium origin, holding it impossible that a substance with a vapour density forty times that of hydrogen should mount to at least equal elevations above the sun’s surface. Sir Norman Lockyer,[199] on the other hand, maintained them to be characteristic of a subtle dissociation-product of calcium, alleging in support of his view the progressive enfeeblement of the “blue line” of calcium (λ 4227), concurrently with the enhancement of H and K, as the substance was more and more completely decomposed at each addition of intensity to the electric current transmitted through the vacuum tubes. The case for dissociation appeared strong; it has, nevertheless, broken down. Sir William and Lady Huggins in 1897 successfully reduced calcium to the “two-line” condition by attenuation alone.[200] Thus the enigmatical prominence-spectrum of calcium was at last artificially produced, and no escape was left from the identification with true metallic calcium of the form of matter encompassing the sun with violet radiance.

A formidable problem, however, remained. Calcium near the sun seems to possess a _levitating_ faculty altogether inexplicable. It floats as high up as hydrogen, or even overtops it. H and K are the most diffusive of all the prominence-rays; they are derived from the summits of the tallest flames, and from every fibre of their texture. This anomalous agility in a comparatively heavy metal must, according to the late Professor Keeler,[201] be the index to some remarkable property unrecognised by ordinary chemical methods. Unless, indeed, something analogous to electrolytic action be in question. There is much to be said in favour of M. Deslandres’s opinion that the chromosphere is electrically luminous;[202] and if so, then “ions,” not molecules or atoms, are presumably its constituting particles. But ions are on a Lilliputian scale of magnitude, and they may be of nearly the same mass for all the chemical elements. Here, however, we trench upon a region of pure speculation. A region, nevertheless, that is likely ere long to be annexed, in part at least, to surveyed territory, since pioneers are actively engaged there. Much, in the interpretation of solar phenomena, depends upon the results of their work; for here, as in every department of astrophysics, the experimental decisions of terrestrial physics must be awaited, not anticipated.

The three ingredients of the chromosphere so far spoken of—hydrogen, helium, and calcium—are found as well in its eruptive outgrowths. Gaseous prominences of all sorts and sizes are thus triply compounded. But we have now to consider a form of matter permanently present in the chromosphere, though rarely projected to any considerable altitude above it. Its badge is a single green ray, which has a curious history. It was momentarily identified with an auroral line; it was long erroneously identified with the distinctive corona-line. It is apparently reversed in the sun—that is to say, a Fraunhofer line falls just in its place. This is Kirchhoff’s “1474” (known as 1474 K), which has proved, under close scrutiny, to be triple. It results from absorption by iron, by cobalt, and by an unrecognised substance. Now the chromospheric ray agrees in position with the iron line, which is one of secondary importance; yet it cannot at present be asserted confidently that it really emanates from glowing iron-vapour. If it did, it should be ordinarily associated with other iron-lines, and none have been ascertained to make part of the fundamental chromospheric spectrum. The vapour giving out “1474 K,” however, is never absent from the solar envelope,[203] although it perhaps subsides at times into its lower strata.[204] On the other hand, it occasionally rises in metallic prominences to a height of about fifteen thousand miles. Mr. Lord, at the Lick Observatory, observed the enigmatical line (λ 5316·8) as shining vividly at the base of a violently disturbed prominence on 4th August 1892,[205] and Mr. Evershed records similar experiences. Dr. Fényi caught a still rarer effect on 19th February 1892 in a prominence attending the great spot-group then visible.[206] The object was peculiar, though not unique, in showing complete forms built up of the various metallic and other substances injected into it from below. Fig. 14 reproduces an instructive drawing made on the spot. The bottom sketch was taken on the C-line when the eruption was at its height. It reached an elevation of 56,000 miles. The overarching of three filaments towards a point at some distance from the base is noteworthy. The second drawing in Fig. 14 is of a date twenty-four minutes later than the first. It depicts the flame in “parhelium” light. The image viewed was constructed on the _red_ ray of “green” helium. This was much smaller than the simultaneous hydrogen-image, and that obtained on the “1474” line had shrunken still further. But it was measured at half-past ten, an hour after the drawing on “C” had been made, when the first vehemence of the outburst had subsided. The line 1474 K is singularly exempt from displacement effects through motion. It remains erect and undeviated in the midst of solar storms. Nor does it widen perceptibly with increase of pressure downward. Its invariable fineness contrasts remarkably with the wedged shape near the photosphere of C, H, and K. Unusual agitation is betokened by its emergence in prominences.

FIG. 14.—Forms of a Prominence in Hydrogen, Helium, and
Pseudo-Coronium (Fényi).
]

Eclipse-spectrographs do not include it, while they have afforded some other quite unexpected results. Of special consequence is Mr. Evershed’s detection of titanium as an unfailing chromospheric element.[207] Plates exposed by him in India, 22nd January 1898, proved to be crowded with ultra-violet lines belonging to this metal. Some among them had indeed been already recognised by Mr. Jewell in Professor Hale’s daylight photographs of prominence-spectra.[208] The height to which they extended indicates a diffusiveness for titanium-vapour equal to that of hydrogen and helium, although inferior to that of calcium. Its atomic weight on the hydrogen scale is 48; it is just as much lighter than iron as it is heavier than calcium. But comparative vapour-densities are, so to speak, impotent for the regulation of elemental distribution near the sun. Another surprise afforded by the Indian eclipse was the conspicuous presence in the chromosphere of scandium as well as of manganese and chromium; while Mr. Hartley[209] has identified as another of its constituents the rare metal gallium from two of its characteristic lines (λ 4172 and λ 4033) recorded during the eclipse of 1893.

On 29th September 1897, Professor Hale, using a grating spectroscope on the forty-inch Yerkes refractor, discerned the green carbon fluting bright at the edge of the sun.[210] Two years later, he found the corresponding yellow band, although the third in the blue remained imperceptible. He inferred the permanent existence near the sun of a shallow layer of carbon vapour.[211] Its rays do not come near the surface; they have to be _dredged for_. Hence the extreme delicacy of detective observations. The anomaly of this behaviour on the part of carbon is glaring. Its specific lightness ought to carry it to altitudes far beyond those attained by titanium and calcium. Yet it lies sunken almost out of sight, while they float manifestly aloft. Some other sorting-out influence besides that of gravity palpably comes into play in the chromosphere and prominences. The possession by the sun of a carbon-envelope, which, although thin visually, must really be at least 500, and may well be 1000 miles in depth, has an important bearing upon the vexed question of photospheric constitution, and tends to strengthen a barely indicated analogy between the sun and a peculiar class of red stars.

The chromospheric spectrum, when its adventitious elements are reckoned in, is highly complex. During a few weeks of 1872, Professor Young, by vigilant watching, determined 273 lines seen, in the clear air of the Rocky Mountains, to flash out intermittently, one by one, or in companies together. And this first systematic enumeration was subsequently greatly extended by its author, while crowds of ultra-violet lines have been added by the photographic investigations of Hale, Deslandres, Evershed, and others. For the most part the rays brightened in eruptions are reversals of Fraunhofer lines, picked out largely at haphazard, yet with an obvious preference, expressly noted by Sir Norman Lockyer, for such as are vivified when the higher excitement of the spark is substituted for the arc in laboratory experiments. This has been taken to imply that the chromospheric is essentially a high-temperature spectrum; but the facts may be differently interpreted.

Among the elements most apt to shine evanescently in metallic prominences are sodium, magnesium, “green” helium, barium, iron, vanadium, and strontium. Gigantic ragged forms, especially when they appear in high latitudes, are of relatively simple chemical composition; or it may be that their condition favours the visibility of only the most persistent radiations. Small compact prominences yield, at any rate, much richer harvests to gleaners of spectroscopic novelties. A specimen of this class was observed by Father Sidgreaves, 10th September 1891.[212] It presented the aspect of “four blow-pipe jets intensely bright at the bends,” was 16,000 miles high, and lasted only an hour. Twenty-six brilliant lines were counted in the visual part of its spectrum, the invisible part, in the absence of suitable appliances, remaining unrecorded. The shape of this flame was equally well defined in _both_ the red rays of helium, and appeared nearly the same in sodium and magnesium. The spectral peculiarities of such objects, however, are most marked in the ultra-violet. A photograph taken at the Kenwood Observatory, 15th October 1892, registered, from an undistinguished prominence, no less than seventy-four bright lines between the wave-lengths 3970 and 3630,[213] most of them being of unknown origin. But since then it has become possible to identify a dozen and upwards with rays measured in the spectra of krypton and xenon by Professors Liveing and Dewar;[214] and the circumstance seems to associate those scarce atmospheric gases with helium as chromospheric constituents.

Occasionally, under circumstances not yet defined or understood, prominences emit continuous light. The sheeny white objects sometimes seen under cover of the moon’s shadow owe their peculiarity to this cause. The whole gamut of prismatic radiance is derived from them, with the addition of bright H and K and a suspicion of hydrogen lines. Thus they are essentially calcium-forms interpenetrated with glowing _dust_. Their light is probably original. If it were reflected, traces of Fraunhofer-absorption which seem to be missing should be perceptible. Distinctively “white” prominences are not common; none were observed during the totality of 28th May 1900. But “red” prominences differ considerably in colour-intensity, all the ruddy shades, from deep ruby to pale pink, being represented in them. Many perhaps consist of a crimson core veiled in almost colourless material. Certainly all are not equally well seen in and out of eclipse. From a comparison of drawings made during the totality of 1870 with his own simultaneous daylight observations, Tacchini inferred that the spectroscope disclosed only the cores of flame in such objects;[215] and the experience was repeated on the occasion of the Egyptian eclipse of 17th May 1882. The four prominences then measured were of a rosy tint, lightening towards the margins, which looked as if edged with a lustrous fillet. “These results,” Professor Hale writes, “may be accepted as establishing an important difference between the spectroscopic and eclipse-images of prominences.” Nevertheless the difference is not constantly present. Some chromospheric forms are identical, viewed prismatically at the edge of the sun, or directly in the dark beside the occulting moon. These are, of course, purely gaseous; the others presumably give a mixture of continuous and discontinuous light.

A very curious feature of the prominence-spectrum was ascertained by Mr. Evershed during the eclipse of 1898.[216] It _becomes_ continuous in the extreme ultra-violet. The range of unbroken radiance begins abruptly just where the hydrogen series ends (near λ 3668), and extends to the limit of the plate’s sensitiveness. Not even a guess can be hazarded at the physical condition underlying this radiative vagary. A different cause must be ascribed to certain local outbursts of white light in eruptive prominences.[217] This symptom of disturbance has been interpreted by Professor Hale as follows. “Objects of the kind,” he says,[218] “are closely related to faculæ, and probably rise from them. It thus occasionally happens that a violent eruption carries some of the white-hot particles to a considerable distance above the photosphere. In such a case the prominence gives a continuous spectrum in addition to its bright lines.” The explanation may pass muster, since no better has been offered.

M. Deslandres succeeded in showing, early in 1892,[219] that the sun may, in a restricted sense, be designated a “bright line star”—that is to say, he elicited from the aggregate of its light evidence of gaseous emissions. Treating the sun as a star by admitting into his spectrographic apparatus the whole of its rays simultaneously, he obtained vivid reversals of the violet calcium lines. But this is only possible when the calcium flames crowning faculæ are widely and strongly developed. Ordinarily their emissions are drowned in the surging flood of continuous radiance. But facular maxima recur, coincidently with spot-maxima, once in about eleven years; so that the periodicity of the sun might conceivably be determined by this one feature at distances obliterative of all other signs of disturbance. Not that the sun viewed, say, from Sirius, could with our actual appliances be detected, even at culminating epochs of agitation, as a bright-line star. Some of his fellow-stars, however, may be in a greatly enhanced stage of his condition, and we may learn to follow their vicissitudes by spectrographic observations of the alternate glimmering and fading of fine rays projected upon the deep shadow of their calcium-absorption. Thus the means may be afforded of ascertaining the flow of change in remote and gigantic orbs; and we shall perhaps in the future be better acquainted with the cyclical peculiarities of Capella or Arcturus than with those of our own “particular star.”