Chapter IV: The Reversing Layer

During the eclipse of 22nd December 1870 a new phenomenon came into view. Professor Young of Princeton, New Jersey, was the fortunate observer. With the slit of his spectroscope tangential to the sun’s limb and perpendicular to the moon’s advance, he awaited the moment of second contact. The thin solar crescent narrowed second by second; at last it vanished; then “all at once, as suddenly as a bursting rocket shoots out its stars,” the ordinary Fraunhofer spectrum previously visible was replaced by a serried array of bright lines on a dark background. They seemed a complete reversal of the familiar absorption-rays, and the impression was also conveyed to Mr. Pye, a member of the same party, of “all the dark lines being converted into bright ones.”[51] The “flash” at the edge of the eclipsed sun was not unlooked for. Something of the kind had been anticipated as the due accompaniment of the beginning and end of totality. For Kirchhoff’s explanation of the Fraunhofer lines implied the interposition, between the eye and the sun, of a screen of glowing vapours, which should be separately, if only instantaneously, visible on the withdrawal of daylight glare—visible, that is to say, spectroscopically; with the telescope nothing more distinctive than a silvery shimmer[52] corresponds to the dazzling variotinted fireworks disclosed by the prism.

But their disclosure was not enough; they demanded close investigation. The question is fundamental in solar physical theory whether the flash is the true reversal of the Fraunhofer spectrum, and no conclusive answer could be given to it except by photographic means, visual reports as to the details of so intricate and evanescent an apparition counting for very little. Twenty-six years, however, elapsed before a permanent record of it was secured. The result ensued from a skilfully-timed snapshot by Mr. Shackleton at Novaya Zemlya during the Arctic eclipse of 9th August 1896. He gave an exposure of half a second with a “prismatic camera”—a simple form of spectrograph, destitute both of slit and collimating lens, the employment of which in eclipse work has been vigorously promoted by Sir Norman Lockyer. An impression was thus caught of singular interest and value. We may quote Professor Young’s description of it. “The photograph,” he writes,[53] “shows a long range of several hundred bright curved images, of which there are nearly 250 in the blue portion of the spectrum between F and H. About 25 are much more extensive and conspicuous than the others, and are images of the chromosphere and prominences. They are due to hydrogen, calcium, helium, strontium, and one or two other elements which often appear in the chromosphere. The rest are simply reversals of the Fraunhofer lines, as Mr. Shackleton has shown by developing the flash spectrum into a bright-line spectrum of the usual form (which is easily done by a simple mechanical contrivance), and comparing it with an ordinary dark-line solar spectrum photographed with the same camera and prisms, but with the addition of a collimator and slit. The agreement is practically complete, although there are two or three somewhat conspicuous Fraunhofer lines which are missing in the flash spectrum, probably because they originate, not above the surface of the photosphere, but in its depths, as probably also do the wide, hazy shadings that accompany the H and K lines and some others; but this is a matter for further investigation.”

The solitary success of 1896 was manifolded a year and a half later. “Reversing-layer” photography stood in the forefront of the programme of work for the Indian eclipse of 22nd January 1898, and the documents collected during its hundred seconds of obscurity showed that a complete mastery of the art had been attained. Sir Norman Lockyer and Mr. Fowler, Captain Hills, Mr. Evershed, and Professor Naegamvala secured photographs of the flash, not only in its full development, but also when incipient and vanishing, so that the phenomenon could be traced at leisure throughout its brief phases. Two specimens are reproduced in Plate III., both taken instantaneously by Mr. Evershed with a prismatic camera of 2¼ inches aperture and 36 inches focus.

The conventional straight appearance of the lines results from the employment of a cylindrical lens to give breadth to narrow slices of the curvilinear originals. The upper section represents the spectrum of the last thread of sunlight just as the accustomed dark lines were fading out before the incoming of their bright correlatives. The range is from below H and K—the strong pair to the left—to λ 3350 in the ultra-violet.[54] The lower section corresponds to a moment twenty seconds later, when the continuous light was gone, and vivid rays dominated the field. Amid the throng, twenty-seven members of the hydrogen series are recognisable, and three titanium lines rival them in importance.

Ample materials were provided during the eclipses of 1898, 1900, and 1901 for at least a preliminary discussion as to the true character of the reversing layer, although the difficulties still remaining to be encountered are neither few nor trivial.

PLATE III.

Flash and Cusp Spectra compared. Ultra-Violet Region λ 4100 to λ 3350.
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To begin with, the fact has been ascertained that a shell of mixed incandescent vapours, five or six hundred miles thick, encloses the photosphere on every side. We see it, however, in projection. The line of sight penetrates it tangentially at the edge of the sun, and to an extreme depth near the base of about forty thousand miles. Between the reversing layer and the chromosphere there is no solution of continuity; to some extent, undoubtedly, the lower merges into the upper formation; yet they are essentially distinct. Each has its own spectrum apart, notwithstanding a certain amount of community, apparent, casual, or partial. Thus since the reversing layer is visually accessible only through the enveloping chromosphere, the spectrographic prints taken at sun-and-moon contacts are inevitably composite. They include the chromospheric together with the “flash” rays. Discrimination is, however, facilitated by the notably greater length of the arcs representative of the former corresponding with the higher extension of the substances emitting them. The class of discrepancies between the flash and the Fraunhofer spectrum thus accounted for are discrepancies by excess. The flash includes helium rays; the Fraunhofer spectrum has none. The flash exhibits the complete hydrogen series up to its extreme limit in the ultra-violet; the Fraunhofer spectrum reverses only its visible members. Titanium lines strong in the flash are feeble in the Fraunhofer spectrum; besides other analogous dissimilarities. Yet they do not affect the claim of the reversing layer to be, speaking broadly, the locus of solar absorption. Rather they bring us face to face with the totally different question, Why do the chromospheric gases exercise no appreciable arresting effect upon the light transmitted through them? Later on we shall attempt to answer it; here we need only remark that the rays from the chromosphere cannot be excluded from photographs of the flash. They necessarily appear in them, and it was known beforehand that they had no counterparts in the Fraunhofer spectrum.

There are besides discrepancies by defect. Many solar absorption lines do not show bright at the first and last instants of totality. But this is easily understood. Some are doubtless too faint to assert their presence photographically. Others may be supposed, with the utmost probability, to originate out of sight at the base of the reversing stratum. The shadings of H and K certainly do, for the central lines start out clear, though strong, in the flash, and their hazy appendages are indubitable products of augmented pressure. Nor do the denser vapours rise high enough to make any perceptible display. Mr. Evershed tells us[55] that, while nearly all the metals with atomic weights less than 60 are represented in the sun’s marginal spectrum, none, of which the ultimate particles are heavier than 92, make any assured contributions to it. A formation at least five hundred miles in vertical extent must vary widely between top and bottom both in composition and density. So at least we should reasonably anticipate. In point of fact the changes indicated are surprisingly slight. One criterion is available by which chemical differences can be correlated with differences of depth. This consists in the various lengths of the curved lines representing the emissions of the sundry constituents of the “layer.”[56] Substances attaining high altitudes, like the chromospheric gases, give long arcs because their visibility extends over a wide section of the sun’s circumference, while low-lying materials, illuminating a narrow verge, are characterised by short arcs. They are, moreover, the most difficult to catch as the moon goes by. Now the great majority of the flash lines are of a nearly equal length, corresponding to an arc of about 40° on the solar limb, and this equality implies a considerable approach to uniformity of constitution throughout the greater part of the momentarily exposed layer. But its basal stratum, perhaps not more than a few miles in thickness, should be that most effective in absorption,[57] and it forms a crescent much too fine to be directly seen. Here below, then, down near the photosphere, missing Fraunhofer lines with no apparent corresponding radiations may be produced; nay, _must_, since there is absolutely no evidence of the corresponding light-stoppage taking place in, or above the chromosphere.[58] Mr. Evershed’s conclusion is indeed fully warranted, that “the flash spectrum as a whole appears to represent the upper more extensively diffused portion of a stratum of gas, which, by its absorption, gives the Fraunhofer spectrum.”[59] The appellation “reversing layer” would then seem to be no misnomer, but to indicate correctly the seat of the linear absorption which serves as our alphabet for spelling out the secrets of solar chemistry.

The density of this vaporous envelope is measurable by the “pressure shifts” of the Fraunhofer lines. It would seem to be nowhere _less_ than that of our atmosphere at sea-level; otherwise displacements towards the _blue_ end should occur, and none such are perceptible. Nor, on the other hand, is there proof of its exceeding, even in the lowest depths, three or four times the standard value. So that the increase of pressure downward is exceedingly slow—a fact to be carefully noted. The distinction (already adverted to) between the effects of total and of partial pressure is also most important. Through the former the positions of spectral rays are modified, through the latter their characters. In other words, the shifts of the Fraunhofer lines correspond to the sum of incumbent vapours, while the quantity of each separately present determines their width and diffuseness. Most are associated, by their fineness and sharpness, with _individually_ tenuous substances. The hydrogen lines, for instance, represent, according to Mr. Maunder,[60] a pressure of only one-hundredth of an atmosphere. But the indications in this respect, as in others, vary greatly for the different vapours.

The most refractory substances, such as titanium and vanadium, are volatilised in the reversing layer. It is, then, enormously heated. But it is cooler than the photosphere, since its rays show dark against the vivid background they are projected upon. Now the temperature of the photosphere, by the most authentic recent determination, is about 6600° centigrade,[61] and this marks an upper limit for the temperature of the reversing layer. A lower limit is fixed by the temperature of the electric arc, estimated at 3500°. The much higher grade of the spark is almost certainly not attained. The inverse behaviour of two magnesium lines, first commented upon by Professors Liveing and Dewar,[62] led Dr. Schemer to this conclusion.[63] One at λ 4352 is prominent in the sun and strong in the arc, but fades out in the spark; the other, at λ 4482, of which a mere trace is perceptible in the sun, is a characteristic spark-product. It must, however, be borne in mind that comparative temperatures are subject to great uncertainty where electricity is the exciting agent. Dissentients are even to be found from the broad proposition that the spark is hotter than the arc; nor is it one capable of direct demonstration. Qualifying circumstances come in, and their separate effects are not easily unravelled.

M. Deslandres made some curious experiments at Paris in 1894 in photographing the sun by means of the dusky rays in its spectrum.[64] For their isolation he used a “double slit”; and since they are only comparatively dark, no difficulty was encountered through want of actinic power in the rays dealt with. He thus succeeded in obtaining with each a monochromatic picture of the sun delineated exclusively with emissions from some particular ingredient of the reversing layer. The uniformity of elemental distribution was, by this ingenious device, put to the test. Photographs of the disc, for instance, taken on an iron line might be expected to show different features from those taken on calcium or magnesium lines if local accumulations of those vapours were present; but no divergences of the kind became perceptible. The composition of the absorbing envelope did not seem to vary _regionally_. The investigation, however, was not carried far, and would be worth prosecuting.

The _fact_ of the existence of a true reversing layer may now be looked upon as established; yet the _mode_ of its existence remains in several ways perplexing. The slightness of its absorptive action needs explanation at the outset. One notes with amazement that the miniature atmosphere surrounding an electric arc is equally effective for light-stoppage with this ocean of vapours. Then there is the singular, and perhaps related circumstance that the spectrum from the limb is not more deeply grooved than the spectrum from the central parts of the disc. The results upon light of being sifted through six hundred and through twenty thousand miles of the mixed materials glowing near the sun are virtually the same. Their comparative tranquillity, too, is unexpected. The reversing layer lies between two agitated structures. Beneath are the photospheric clouds, rent and whirling under the stress of cyclonic disturbances; above, the chromospheric flames, driven hither and thither by influences of fantastic violence. Yet a region of peace seems to intervene. The Fraunhofer lines indicate a steady vertical circulation, but scarcely ever a temporary commotion. By a rare exception, Father Fényi observed at Kalocsa, 27th July 1887, the dark C in the neighbourhood of a spot-group, displaced alternately towards the blue and the red, indicating, he supposed, a powerful disturbance of the reversing stratum by an irruption of hot hydrogen.[65] Such invasions of its precincts, however, are under the ban of some prohibitive decree, or they encounter unknown difficulties. They occur, at any rate, with remarkable infrequency.

The reversing layer is heated from below, and gravitates downward. Thermal equilibrium is doubtless maintained by the convective transport of material, but the due effects of superincumbent weight are unapparent. Evidence is not indeed wanting of _some_ increase of density with descent, but of an increase relatively insignificant. Gravity at the sun’s surface possesses nearly twenty-eight times its terrestrial power; hence a true solar atmosphere should double its density with each furlong of approach to the sun’s surface,[66] and the total increase of pressure in an envelope five or six hundred miles deep would be “inexpressible by numbers that have name.” Actually there is, at the most, a quintupling of pressure. This formidable discrepancy is altogether unexplained. We are debarred by it from considering the reversing layer to be in statical equilibrium. Its successive strata do not rest one upon the other under the sole dominion of gravity. Some counteracting influence is brought to bear. This problem of _levity_—so to call it—is one that perpetually recurs in studying the solar surroundings.