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:—