Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder.
The Corona is perhaps the most mysterious of all the sun's surroundings. As yet its nature remains undetermined, though the observations which have been made at every eclipse since attention was first directed to it have been gradually suggesting and strengthening the idea that there exists a very close analogy between the coronal streamers and the Aurora or the tails of comets. The extreme rarity of its substance is conclusively proved by the fact that such insubstantial things as comets pass through it apparently unresisted and undelayed. Its structure presents variations in different latitudes. Near the poles it exhibits the appearance of brushes of light, the rays shooting out from the sun towards each summit of his axis, while the equatorial rays curve over, presenting a sort of fish-tail appearance. These variations are modified, as already mentioned, by some cause which is at all events coincident with the sun-spot period. At minimum the corona presents itself with polar brushes of light and fish-tail equatorial rays, the latter being sometimes of the most extraordinary length, as in the case of the eclipse of July 29, 1878, when a pair of these wonderful streamers extended east and west of the eclipsed sun to a distance of at least 10,000,000 miles.
When an eclipse occurs at a spot-maximum, the distribution of the coronal features is found to have entirely changed. Instead of being sharply divided into polar brushes and equatorial wings, the streamers are distributed fairly evenly around the whole solar margin, in a manner suggesting the rays from a star, or a compass-card ornament. The existence of this periodic change has been repeatedly confirmed, and there can be no doubt that the corona reflects in its structure the system of variation which prevails upon the sun. 'The form of the corona,' says M. Deslandres, 'undergoes periodical variations, which follow the simultaneous periodical variations already ascertained for spots, faculæ, prominences, and terrestrial magnetism.' Certainty as to its composition has not yet been attained; nor is this to be wondered at, for the corona is only to be seen in the all too brief moments during which a total eclipse is central, and then only over narrow tracts of country, and all attempts to secure photographs of it at other times have hitherto failed. When examined with the spectroscope, it yields evidence that its light is derived from three sources—from the incandescence of solid or liquid particles, from reflected sunshine, and from gaseous emissions. The characteristic feature of the coronal spectrum is a bright green line belonging to an unknown element which has been named 'coronium.'
The Chromosphere and the Prominences, unlike the elusive corona, may now be studied continuously by means of the spectroscope, and instruments are now made at a comparatively moderate price, which, in conjunction with a small telescope—3 inches will suffice—will enable the observer to secure most interesting and instructive views of both. The chromosphere is, to use Miss Clerke's expression, 'a solar envelope, but not a solar atmosphere.' It surrounds the whole globe of the sun to a depth of probably from 3,000 to 4,000 miles, and has been compared to an ocean of fire, but seems rather to present the appearance of a close bristling covering of flames which rise above the surface of the visible sun like the blades of grass upon a lawn. Any one of these innumerable flames may be elevated into unusual proportions in obedience to the vast and mysterious forces which are at work beneath, and then becomes a prominence. On the whole the constitution of the chromosphere is the same as that of the prominences. Professor Young has found that its normal constituents are hydrogen, helium, coronium, and calcium. But whenever there is any disturbance of its surface, the lines which indicate the presence of these substances are at once reinforced by numbers of metallic lines, indicating the presence of iron, sodium, magnesium, and other substances.
The scale to which these upheavals attain in the prominences is very remarkable. For example, Young records the observation of a prominence on October 7, 1880. When first seen, at about 10.30 a.m., it was about 40,000 miles in height and attracted no special attention. Half an hour later it had doubled its height. During the next hour it continued to soar upwards until it reached the enormous altitude of 350,000 miles, and then broke into filaments which gradually faded away, until by 12.30 there was nothing left of it. On another occasion he recorded one which darted upwards in half an hour from a moderate elevation to a height of 200,000 miles, and in which clouds of hydrogen must have been hurled aloft with a speed of at least 200 miles per second. (Plate [IX.] gives a representation of the chromosphere and prominences from a photograph by M. Deslandres.) Between the chromosphere and the actual glowing surface of the sun which we see lies what is known as the 'reversing layer,' from the fact that owing to its presence the dark lines of the solar spectrum are reversed in the most beautiful way during the second at the beginning and end of totality in an eclipse. Young, who was the first to observe this phenomenon (December 22, 1870), remarks of it that as soon as the sun has been hidden by the advancing moon, 'through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by hundreds and thousands, almost startlingly; as suddenly as stars from a bursting rocket-head, and as evanescent, for the whole thing is over within two or three seconds.'
The Chromosphere and Prominences, April 11, 1894. Photographed by M. H. Deslandres.