Auroræ: Their Characters and Spectra. J. Rand Capron
Aurora of 1716, occurring after an interval of eighty years, which Dr. Halley describes, was very brilliant and extended over much country, being seen from the west of Ireland to the confines of Russia and the east of Poland, extending nearly 30° of longitude, and from about the 50th degree of latitude, over almost all the north of Europe, and in all places exhibiting at the same time appearances similar to those observed in London. An Aurora observed in Bologna in 1723 was stated to be the first that had ever been seen there; and one recorded in the ‘Berlin Miscellany’ for 1797 is called a very unusual phenomenon. Nor did Auroræ appear more frequent in the Polar Regions at that time, for Cælius states that the oldest inhabitants of Upsala considered the phenomenon as quite rare before 1716. Anderson, of Hamburg, writing about the same time, says that in Iceland the inhabitants themselves were astonished at the frequent Auroræ then beginning to take place; while Torfæus, the Icelander, who wrote in 1706, was old enough to remember the time when the Aurora was an object of terror in his native country.
According to M. Mairan, 1441 Auroræ were observed between A.D. 583 and 1751, of which 972 were observed in the winter half-years and 469 during the summer half-years. In our next Chapter we propose to give some general descriptions of Auroræ from comparatively early sources.
CHAPTER II.
SOME GENERAL DESCRIPTIONS OF AURORÆ.
Sir John Franklin’s description.
Sir John Franklin (‘Narrative of a Journey to the Shores of the Polar Sea in the years 1819, 1820, 1821, 1822’) describes an Aurora in these terms:—
Parts of the Aurora: beams, flashes, and arches.
“For the sake of perspicuity I shall describe the several parts of the Aurora, which I term beams, flashes, and arches.
“The beams are little conical pencils of light, ranged in parallel lines, with their pointed extremities towards the earth, generally in the direction of the dipping-needle.
Formation of the Aurora.
“The flashes seem to be scattered beams approaching nearer to the earth, because they are similarly shaped and infinitely larger. I have called them flashes, because their appearance is sudden and seldom continues long. When the Aurora first becomes visible it is formed like a rainbow, the light of which is faint, and the motion of the beams undistinguishable. It is then in the horizon. As it approaches the zenith it resolves itself into beams which, by a quick undulating motion, project themselves into wreaths, afterwards fading away, and again and again brightening without any visible expansion or contraction of matter. Numerous flashes attend in different parts of the sky.”
Arches of the Aurora.
Sir John Franklin then points out that this mass would appear like an arch to a person situated at the horizon by the rules of perspective, assuming its parts to be equidistant from the earth; and mentions a case when an Aurora, which filled the sky at Cumberland House from the northern horizon to the zenith with wreaths and flashes, assumed the shape of arches at some distance to the southward. He then continues:—“But the Aurora does not always make its first appearance as an arch. It sometimes rises from a confused mass of light in the east or west, and crosses the sky towards the opposite point, exhibiting wreaths of beams or coronæ boreales on its way. An arch also, which is pale and uniform at the horizon, passes the zenith without displaying any irregularity or additional brilliancy.” Sir John Franklin then mentions seeing three arches together, very near the northern horizon, one of which exhibited beams and even colours, but the other two were faint and uniform. (See example of a doubled arc Aurora observed at Kyle Akin, Skye, Plate VII.)
He also mentions an arch visible to the southward exactly similar to one in the north. It appeared in fifteen minutes, and he suggests it probably had passed the zenith before sunset. The motion of the whole body of the Aurora from the northward to the southward was at angles not more than 20° from the magnetic meridian. The centres of the arches were as often in the magnetic as in the true meridian. A delicate electrometer, suspended 50 feet from the ground, was never perceptibly affected by the Aurora.
Aurora does not often appear until some hours after sunset.
Sir John Franklin further remarks that the Aurora did not often appear immediately after sunset, and that the absence of that luminary for some hours was in general required for the production of a state of atmosphere favourable to the generation of the Aurora.
Aurora seen in daylight.
On one occasion, however (March 8th, 1821), he observed it distinctly previous to the disappearance of daylight; and he subsequently states that on four occasions the coruscations of the Aurora were seen very distinctly before daylight had disappeared.
[In the article “Aurora Polaris,” Encyc. Brit. edit. ix., the Transactions of the Royal Irish Academy, 1788, are referred to, where Dr. Usher notices that the Aurora makes the stars flutter in the telescope; and that, having remarked this effect strongly one day at 11 A.M., he examined the sky, and saw an Auroral corona with rays to the horizon.
Instances are by no means rare of the principal Aurora-line having been seen in waning sunlight, and in anticipation of an Aurora which afterwards appeared.]
The Rev. James Farquharson’s observations. Auroral arch. Passage across the zenith.
The Rev. James Farquharson, from the observation of a number of Auroræ in Aberdeenshire in 1823 (‘Philosophical Transactions,’ 1829), concluded:—that the Aurora follows an invariable order in its appearance and progress; that the streamers appear first in the north, forming an arch from east to west, having its vertex at the line of the magnetic meridian (when this arch is of low elevation it is of considerable breadth from north to south, having the streamers placed crosswise in relation to its own line, and all directed towards a point a little south of the zenith); that the arch moves forward towards the south, contracting laterally as it approaches the zenith, and increasing its intensity of light by the shortening of the streamers and the gradual shifting of the angles which the streamers near the east and west extremities of the arch make with its own line, till at length these streamers become parallel to that line, and then the arch is seen in a narrow belt 3° or 4° only in breadth, stretching across the zenith at right angles to the magnetic meridian; that it still makes progress southwards, and after it has reached several degrees south of the zenith again enlarges its breadth by exhibiting an order of appearances the reverse of that which attended its progress towards the zenith from the north; that the only conditions that can explain and reconcile these appearances are that the streamers of the Aurora are vertical, or nearly so, and form a deep fringe which stretches a great way from east to west at right angles to the magnetic meridian, but which is of no great thickness from north to south, and that the fringe moves southward, preserving its direction at right angles to the magnetic meridian.
M. Lottin’s observations.
Dr. Lardner, in his ‘Museum of Science and Art,’ vol. x. p. 189 et seq., alludes to a description of “this meteor” (sic) supplied by M. Lottin, an officer of the French Navy, and a Member of the Scientific Commission to the North Seas. Between September 1838 and April 1839, being the interval when the sun was constantly below the horizon, this savant observed nearly 150 Auroræ. During this period sixty-four were visible, besides many concealed by a clouded sky, but the presence of which was indicated by the disturbances they produced upon the magnetic needle.
The succession of appearances and changes presented by these “meteors” is thus graphically described by M. Lottin:—
Formation of the auroral bow.
“Between four and eight o’clock P.M. a light fog, rising to the altitude of six degrees, became coloured on its upper edge, being fringed with the light of the meteor rising behind it. This border, becoming gradually more regular, took the form of an arch, of a pale yellow colour, the edges of which were diffuse, the extremities resting on the horizon.