II. AS we have said, it was net uncommon at the Vega's winter quarters to see two or more auroral-arcs, one of which was generally the “common arc.” The second was nearly parallel to it and separated from it by an unlighted space which was sometimes crossed by rays of light. It would be most important for a thorough knowledge of auroræ to know the true mutual position of the arcs; but here again simultaneous measurements at two distant places are necessary, and not having such, Nordenskjöld remarks that three suppositions may be made. First, that the two arcs have irregular positions with regard to one another; secondly, that they are superposed on one another, having their centres on the same axis perpendicular to the surface of the earth; and third, that their centres are on the same radius of the earth, and that they are situated in about the same plane. In all three cases the aspects of the arcs would be quite different. The observations at the Vega's wintering place prove that the last case is the rule, and that arcs irregularly situated with report to one another, or crossing one another (which would correspond to the first and second supposition), are exceptions; and Nordenskjöld arrives at the conclusion that the auroræ-arcs which were seen from the Vega, were usually in about the sarre plane. It might be asked, however, if it were not more natural to suppose that both rings are at the same distance from the earth's surface, their centres being situated on the same radius of the earth? But on March 14 two parallel arcs appeared, and soon joined together into a broad belt, the interior edge of which was 5°, and the exterior one was 15° above the horizon, both edges being quite concertric, and the belt showing a tendency to divide into parallel bands, whilbt its brilliancy remained the same towards the lower and upper edges; Nordenkjöld considers therefore as most probable that all the luminous sheet afforded by these arcs was in the same plane.
A. E. Nordenskjöld, "Om norrskenen under Vegas öfvervintring vid Berings Sund, 1878–79," in "Vega Expeditionen Vetenskapliga Arbeten." (The Scientific Work of the Vega Expedition, part 1, pp. 401–452.)
"Die Nordlicht Beobachtungen der Österr. Ung. Arct. Exped.," in the "Denkschriften der Math.-Wiss. Classe der k. Acad. der Wiss., Wien," Bd. xxxv., 1878.