CHAPTER VIII.

AURORA AND THE SOLAR CORONA.

[Sidenote: Aurora and the solar corona. Mr. Norman Lockyer’s ‘Solar Physics.’]

Mr. Norman Lockyer, in his ‘Solar Physics,’ a work of 666 pages, gives but little space to the Aurora. The index comprises:—“Aurora Borealis, connexion with sun-spots, pp. 82-102.” “Affirmed coincidence of spectrum with that of the corona, pp. 244, 256.”

[Sidenote: Extracts from as to Aurora’s connexion with sun-spots and with solar corona.]

Page 82. After referring to Gen. Sabine as having shown that there are occasional disturbances in the magnetic state of the earth, and that these disturbances have a periodical variation, coinciding in period and epoch with the variation in frequency and magnitude of the solar spots as observed by Schwabe, the author proceeds to state, “and the same philosopher has given us reason to conclude that there is a similar coincidence between the outburst of solar spots and of the Aurora Borealis.”

Page 102. “We have also shown that sun-spots or solar disturbances appear to be accompanied by disturbances of the earth’s magnetism, and these again by auroral displays.”

[Sidenote: Evidence of American observers on nature of the corona considered.]

Page 243. “What, then, is the evidence furnished by the American observers on the nature of the corona (solar)? It is bizarre and puzzling to the last degree. The most definite statement on the subject is that it is nothing more nor less than a _permanent Solar Aurora_! the announcement being founded on the fact that three bright lines remained visible after the image of a prominence had been moved away from the slit, and that one (if not all) of these lines is coincident with a line (or lines) noticed in the spectrum of the Aurora Borealis by Professor Winlock.” Mr. Lockyer then adds, that amongst the lines he had observed up to that time, some forty in number, this line was among those which he had most frequently recorded, and was, in fact, the first iron line which made its appearance in the part of the spectrum he generally studied, when the iron vapour is thrown into the chromosphere.

[Sidenote: Mr. Lockyer’s conclusion adverse to the question being settled.]

Hence he thought he should always see it if the Aurora were a permanent solar corona, and gave out this as its brightest line, and on this ground alone should hesitate to regard the question as settled.

[Sidenote: Prof. Young’s communication to ‘Nature.’]

Page 256 is an extract from a communication by Prof. Young to ‘Nature,’ March 24, 1870, in which the Professor refers to the bright line 1474 as being always visible with proper management. He also thinks it probable that this line coincides with the Aurora line reported by Prof. Winlock at 1550 of Dr. Huggins’s scale, though he is by no means sure of it. He had only himself seen it thrice, and then not long enough to complete a measurement. He was only sure that its position lay between 1460 and 1490 of Kirchhoff.

[Sidenote: He does not abandon his hypothesis, it having other elements of probability.]

For this reason he did not abandon the hypothesis, which appeared to have other elements of probability, in the general appearance of the corona, the necessity of immense electrical disturbances in the solar atmosphere as the result of the powerful vertical currents known to exist there, as well as the curious responsiveness of our terrestrial magnets to solar storms; yet he did not feel in a position to urge it strongly, but rather awaited developments. Father Secchi was disposed to think the line hydrogen, while Mr. Lockyer still believed it to be iron.

[Sidenote: Dr. Schellen reviews the subject in eclipse of 1869.]

Dr. Schellen, in his ‘Spectrum Analysis,’ treats the matter more in detail. Referring to the eclipse of 1869 as confirming the previous observations that the coronal spectrum was free from dark lines, he points out that Pickering, Harkness, Young, and others were agreed that with the extinction of the last rays of the sun all the Fraunhofer lines disappeared at once from the spectrum. He further says:—

[Sidenote: Young observed three bright lines in the spectrum of the corona. Coincidence of these lines with three bright lines observed by Winlock in the Aurora. Corona self-luminous, and probably of a gaseous nature. Corona supposed to be a permanent polar light existing in the sun. Polar light in the sun attributed to electricity. Dr. Schellen thinks nature of the corona still a problem.]

“The small instruments employed by Pickering and Harkness, with a large field of view, exhibited a spectrum obtained at once from the corona, the prominences, and the sky in the neighbourhood of the sun. These instruments showed during totality a faint continuous spectrum free from dark lines, but crossed by two or three bright lines. Young, with a spectroscope of five prisms, observed the three bright lines in the spectrum of the corona, and deduced the following positions according to Kirchhoff’s scale:—1250 ± 20, 1350 ± 20, and 1474. It had been already explained why the last and brightest of these lines was thought to belong to the corona and not to that of the prominences, and it seemed probable that the other two lines belonged also to the light of the corona, from the fact that they were both wanting in the spectrum of the prominences when observed without an eclipse. But what invested these three lines with a peculiar interest was the circumstance that they appeared to coincide exactly with the first three of the five bright lines observed by Professor Winlock in the spectrum of the Aurora Borealis. These lines of the Aurora were determined by Winlock according to Huggins’s scale; and if these be reduced to Kirchhoff’s scale, the positions of the lines would be 1247, 1351, and 1473, while the lines observed by Young were 1250, 1350, and 1474.” Dr. Schellen then points out that if it be borne in mind that Young found the positions of the two fainter lines more by estimation than by measurement, the coincidence between the bright lines of the corona and those of the Aurora would be found very remarkable. The brightest of the lines, 1474, was the reversal of a strongly marked Fraunhofer line, ascribed by Kirchhoff and Ångström to the vapour of iron. Dr. Schellen then details Professor Pickering’s observations with the polariscope, showing that the corona must be self-luminous, and that from the bright lines seen in its spectrum it is probably of a gaseous nature, and forms a widely diffused atmosphere round the sun; and then adds, “It has been supposed, from the coincidence of the three bright lines of the corona with those of the Aurora Borealis, that the corona is a permanent polar light existing in the sun analogous to that of our earth.” Dr. Schellen here adds:—“Lockyer, however, justly urges against this theory the fact that although the brightest of these three lines, which is due to the vapour of iron, is very often present among the great number of bright lines occasionally seen in the spectrum of the prominences, it is by no means constantly visible, which ought to be the case if the corona were a permanent polar light in the sun.” (Professor Young’s answer to this, on the ground of line 1474 being always visible, has been already given.) “A yet bolder theory is the ascription of such a polar light in the sun to the influence of electricity, which has been proved, it is well known, by the relation of the magnetic needle, and the disturbance of the electric current in the telegraph wires, to play an important part in the phenomenon of the Aurora Borealis;” and Dr. Schellen then concludes with an opinion that the nature of the corona was still a problem[12].

[Sidenote: Various places of wave-length assigned to these lines.]

On reference to the ‘American Journal of Science,’ vol. xlviii. pp. 123 and 404, it seems that the auroral observations before referred to were made on 15th April, 1869, by C. S. Pierce, with “an ordinary chemical spectroscope, with the collimator pointed directly to the heavens,” and were reported by Winlock. The lines were 1280, 1400, and 1550 of Huggins’s scale, and were reduced to Kirchhoff’s scale by Young. These lines have had all sorts of places of wave-length assigned to them by different writers. Proctor gives 5570, 5400, 5200; Pickering and Alvan Clarke, 5320 (assumed to be 5316, coronal line); Barker, 5170, 5200, 5020; Backhouse, 5320, 4640, and 4310. In my ‘Aurora Spectrum,’ Plate XII., I have assigned two, with a?, to 5320 (Alvan Clarke) and 5020 (Barker). The third might perhaps be placed at 4640 (Backhouse and Winlock).

[Sidenote: Doubts raised as to closeness of the observations for the purpose of comparison.]

The coincidences relied on in the foregoing observations depend, of course, upon (1) the accuracy of the observations themselves, and (2) the subsequent reduction of the lines for comparison. Assuming the correctness of the latter, what have we as to the former? Two of Professor Young’s positions of coronal lines, as stated, seem to have far too much of the ± element to make them sufficiently accurate. Pierce’s auroral observation does not state how the lines were positioned. As they _all_ end with a cypher, the suspicion naturally arises that the measurements did not extend beyond the first three places of the figures, and, if so, could not be used for accurate comparison. The auroral lines, too, are generally rather wide and nebulous, and not easy of comparison with sharper ones.