« PreviousContinue »
STRANGE NEWS ABOUT THE SOLAR
BY RICHARD A, PROCTOR, B.A., F.R.A.S.
on their due positfacts are indeecable at
NUR knowledge respecting the sun has increased so rapidly
of late, that it is by no means easy for the astronomer to place in their due position all the facts which have become known. Some of these facts are indeed altogether strange and unexpected; they seem almost inexplicable at a first view, and the more carefully they are studied the more striking do they appear. Quite recently we have received from two different sources the narrative of observations which bear in a most important manner on the interpretation of solar phenomena. From Fr. Secchi, of Rome, we receive the records of a long and careful series of researches, confirming the startling announcements made by Zöllner and Respighi, and adding other information of extreme interest. From Professor Young, of America, we have the account of a single solar outburst, but the most wonderful by far that has yet been witnessed, and affording highly significant evidence respecting the mighty forces at work in the sun's globe.
I propose to consider, here, the bearing of the information thus recently obtained, not merely on the subject of the solar prominences, but on those questions respecting the physical condition of the sun's globe on the one hand, and the nature of the corona on the other, which have recently attracted so much attention. For I conceive that the great fact which is becoming more and more clearly discerned as observation progresses is this, that the phenomena presented by the sun's globe, or rather by the photosphere we see, are intimately associated with the phenomena presented by the solar corona; and that the bond of union thus associating the two series of phenomena is to be recognised in the processes at work in the coloured envelope,—the sierra or chromatosphere,—which mav be regarded as one of the solar atmospheres. We are waiting at present for further information on this very point from the observers of the eclipse of December 12; but beyond all question very clear information was obtained during the Mediterranean eclipse of December 1870. Spectroscopy and polariscopy did not avail to tell us all we wished to know respecting the corona ; and through unfavourable weather photography failed in doing what it would assuredly have done had the sky at Syracuse cleared round the sun only two minutes earlier. In the last eleven seconds of totality, however, one good picture of the corona (the first ever taken) was obtained by Mr. Brothers; and that picture, besides showing what Mr. Brothers's method was capable of effecting, gave evidence of the utmost importance in relation to the physical condition of the sun. Combined with the spectroscopic charting of the prominences by Mr. Seabroke (during the day of the eclipse, but not during totality), and confirmed by the photograph taken in Spain by Mr. Willard, as well as by the direct observation of the inner corona by Professor Watson, this photograph indicates an association between the prominences, the inner corona and the outer radiated corona, which must be accounted for in any theory respecting the condition of the matter surrounding the sun's globe. Wherever the prominences were large and remarkable, there the inner corona was brightest and extended farthest from the sun, and opposite those same regions lay the great radial beams of the outer corona. Combining these relations with the well-known fact that the solar spot-zone is the region in which the prominences have their greatest activity, we see that we are on the traces of a law relating to the whole economy of the great ruling luminary of our planetary system.
Now the study of the solar spots, on the one hand, presents difficulties so serious in their nature that we can scarcely wonder at the fact that hitherto no consistent theory has been put forward in explanation of their phenomena; and, on the other hand, the study of the solar corona is simply the most difficult of all the subjects of investigation which the student of solar physics can present to himself. Holding a place between the phenomena of the spots and those presented by the corona, and associating together these classes of phenomena, are the phenomena presented by the prominences; and these can fortunately be studied in a systematic and (all things considered) a satisfactory manner. So long as the prominences could be studied only during eclipses, it was almost hopeless to look to them for information respecting the difficult problems of solar physics ; but so soon as a method was devised for examining their features when the sun is not eclipsed, the whole subject of solar research assumed a new aspect. Since that day the progress of discovery has been so rapid as to render it difficult to believe that the method was first applied only three years ago.
Passing over the first observations of Janssen, Lockyer, Capt. J. Herschel, and Secchi, and giving less attention to the questions of the condition of the prominences as respects temperature and pressure than to the motions of the prominence matter, we find in the work of Zöllner and Respighi the first clear intimations of the wonderful activity of the glowing vapours surrounding the sun's globe. So far back as the spring of 1869, Zöllner recognised the action of solar repulsive forces
—which he regarded and still regards as eruptive--in casting forth enormous masses of glowing hydrogen. In several papers he has discussed the evidence he has obtained respecting the energy of these forces, arriving at conclusions which were regarded at the time as startling in the extreme, but must now be considered as falling far short of the reality. He assigned 120 miles per second as the probable velocity of outrush in solar eruptions, and spoke of eighty or ninety thousand miles as the probable limit of height to which the erupted matter attains before, gradually descending, it spreads itself into the strange forms constituting the cloud-like as distinguished from the eruptive prominences.
Respighi was led to regard the repulsive action of the sun as electrical in origin; but as he agrees with Zöllner in regarding the prominences as solar eruptions, it is a matter of comparatively small importance that he considers the force producing the eruptions as something very different in its nature from the volcanic action believed in by Zöllner. At the present stage of our progress it is much more important to determine the extent and energy of the solar eruptions than the cause or causes to which they may be due. Respighi gave the following account of the appearances presented by the prominences. It is important that his description should be carefully attended to, as it supplies independent evidence of some of the remarkable observations made by Father Secchi. 66 When there are faculæ on the sun there are usually prominences; but over the sunspots themselves, though there are low jets, there are no high prominences. As respects the distribution of prominences round the sun's limb, it is to be noticed that great prominences are never recognised in the circumpolar solar regions, and the prominences actually seen, besides being small, are few in number, and last but a short time. At the solar equator the prominences are less frequent, less active, and less developed than in higher solar latitudes." He found that “the formation of a prominence is usually preceded by the appearance of a rectilinear jet, either vertical or oblique, and very bright and well defined. This jet rising to a great height is seen to bend back again, falling upon the sun like the jets of our fountains, and presently the sinking matter is seen to assume the shape of gigantic trees, more or less rich in branches and foliage. Gradually the whole sinks down upon the sun, sometimes forming isolated clouds before reaching the solar surface. It is in the upper portions of such prominences that the most remarkable and rapid transformations are witnessed; but a great difference is observed in the rate with which prominences change in figure. Their duration, also, is very variable. Some develope and disappear in a few minutes, while others remain visible for several successive days."
Respighi agrees with Zöllner in considering that the wellmarked bases of the eruptive jets“ proves that the eruption takes place through some compact substance forming a species of solar crust," and also in believing “ that the enormous velocity with which these gaseous masses rush through the solar atmosphere implies that the latter is of excessive tenuity." The highest prominence observed by Respighi had an elevation of no less than 160,000 miles.
Secchi's recent researches, or the researches he has recently completed, result in a classification of the whole series of phenomena presented by the sierra and the prominences. In the first place, he remarks that the sierra or chromatosphere presents four distinct aspects. At times it has a perfectly smooth and well-defined outline, and is very little less brilliant at the edge than throughout the remaining portion of its depth. At other times, though the chromatosphere is quite smooth, and as it were calm, its brilliancy diminishes outwards so gradually that no limit can be distinguished; more frequently the sierra is surmounted by filaments all sloped in the same direction. And lastly, and most frequently of all, the chromatosphere has an irregular outline, and is fringed with small tongues of flame having no specific direction.
The prominences may be divided into three general ordersheaps, jets, and plumes.
The heaped prominences are of three kinds. First, there are slight elevations of the corona rarely more than 15 or 20 seconds in height, and having an outline either diffuse like the second form of the chromatosphere, or fringed like the third or fourth forms of that layer. · Secondly, there are brilliant masses resembling our cumulus clouds. Thirdly, there are large diffuse masses suspended above or attached to the tops of the larger prominences.
Next in order are the jets, the most interesting of all the prominences on account of the evidence they afford of mighty repulsive or eruptive forces.
2 the form such as are shones often a gre
Some of the jets are small, quickly variable in shape, and last but a short time. They resemble, in fact, as pictured by Secchi, a mere development and extension of the irregularities seen in the fourth form of the chromatosphere.
Next in order are jets such as are shown in fig. 1 of the illustrative plate. Such jets are not often met with on a great scale. Secchi terms them cones. Such cones often extend themselves into curved shapes such as are shown in fig. 2; the transformation from the form shown in fig. 1 to that of fig. 2 occupying only about twenty minutes. Nor is the transformation gradual, but one form passes quickly into the other after a short interval of seeming tranquillity. “The luminosity of jets is always very great," says Secchi, “their roots being more luminous than the rest of the solar surface.* Their appearance is extremely beautiful; the most splendid display of fireworks would fall far short of realising to the imagination the magnificent glory of the sublime spectacle they present. Sometimes the branches fall in the shape of parabolas more or less inclined ; at other times they are like the heads of immense palms with the most graceful curving branches.” In figs. 3 and 4 of the plate are shown some of the forms assumed by these jets. “The branches," says Secchi, “incline sometimes in the direction of the jet, sometimes recoil upon the stalk from which they spring. This kind of jet is always compact, filamentary to the base, and terminated at the apex without any clear, decided outlines. Their light is so bright that they can be seen through the light clouds into which the chromatosphere breaks up. Their spectrum indicates besides hydrogen the presence of many other substances.” (The italics are mine, and I invite special attention to the statement here made by Fr. Secchi.) “ These I call sheaves. I frequently observe in sheaves a great variability in the refrangibility of the rays” (that is, the indications of very rapid motions). “Frequently also, when they have attained a certain height, they cease to grow, and become transformed into exceedingly brilliant masses, which after a time separate and form fiery clouds. A characteristic of sheaves as of the flames is their short duration; they rarely last an hour, frequently only a few minutes.” .
The prominences of the third class-called plumes by Secchi-resemble the jets in some respects, but differ from them in being less bright, and in remaining longer visible; in having their extremities sometimes surmounted by or resolved
• Secchi here refers, of course, to the appearance presented in the spectroscope. If the jets were in any part of their extent actually brighter than the sun's surface, they would be visible without spectroscopic aid; which has never happened.