whether an atmospheric light is aurora. The Swedish Expedition (17) of 1899–1902, engaged in measuring an arc of the meridian in Spitsbergen, were unusually well provided spectrographically, and succeeded in taking photographs of aurora in conjunction with artificial lines—chiefly of hydrogen—which led to results claiming exceptional accuracy. In the spectrograms three auroral rays—including the principal one mentioned above—were pre-eminent. For the two shorter wave-lengths, for whose measurement he claims the highest precision, the observer, J. Westman, gives the values 4276·4 and 3913·5. In addition, he assigns wave-lengths for 156 other auroral lines between wave-lengths 5205 and 3513. The following table gives the wave-lengths of the photographically brightest of these, retaining four significant figures in place of Westman’s five.
| 4830 4709 4699 4661 4560 4550 |
4489 4420 4371 4356 4344 4337 |
4329 4242 4230 4225 4078 4067 |
3997 3986 3947 3937 3880 3876 |
3861 3804 3793 3704 3607 3589 |
There are a number of optically bright lines of longer wave-length. For the principal of these Angot (1) gives the following wave-lengths (unit 1 µµ or 1 × 10−9 metre):—630, 578, 566, 535, 523, 500.
Out of a total of 146 auroral lines, with wave-lengths longer than 3684 tenth-metres, Westman identifies 82 with oxygen or nitrogen lines at the negative pole in vacuum discharges. Amongst the lines thus identified are the two principal auroral lines having wave-lengths 4276·4 and 3913·5. The interval considered by Westman contains at least 300 oxygen and nitrogen lines, so that approximate coincidence with a number of auroral lines was almost inevitable, and an appreciable number of the coincidences may be accidental. E. C. C. Baly (21), making use of the observations of the Russian expedition in Spitsbergen in 1899, accepts as the wave-lengths of the three principal auroral lines 5570, 4276 and 3912; and he identifies all three and ten other auroral lines ranging between 5570 and 3707 with krypton lines measured by himself. In addition to these, he mentions other auroral lines as very probably krypton lines, but in their case the wave-lengths which he quotes from Paulsen (22) are given to only three significant figures, so that the identification is more uncertain. The majority of the krypton lines which Baly identifies with auroral lines require for their production a Leyden jar and spark gap.
If, as is now generally believed, aurora represents some form of electrical discharge, it is only reasonable to suppose that the auroral lines arise from atmospheric gases. The conditions, however, as regards pressure and temperature under which the hypothetical discharges take place must vary greatly in different auroras, or even sometimes in different parts of the same aurora. Further, auroras are often possessed of rapid motion, so that conceivably spectral lines may receive small displacements in accordance with Doppler’s principle. Thus the differences in the wave-lengths of presumably the same lines as measured by different Arctic observers may be only partly due to unfavourable observational conditions. Many of the auroral lines seen in any single aurora are exceedingly faint, so that even their relative positions are difficult to settle with high precision.
24. Whether or not auroral displays are ever accompanied by a characteristic sound is a disputed question. If sound waves originate at the seat of auroral displays they seem hardly likely to be audible on the earth, unless the aurora comes very low and great stillness prevails. It is thus to the Arctic one looks for evidence. According to Captain H. P. Dawson (26), in charge of the British Polar Station at Fort Rae in 1882–1883, “The Indians and voyageurs of the Hudson Bay Company, who often pass their nights in the open, say that it [sound] is not uncommon . . . there can be no doubt that distinct sound does occasionally accompany certain displays of aurora.” On the one occasion when Captain Dawson says he heard it himself, “the sound was like the swishing of a whip or the noise produced by a sharp squall of wind in the upper rigging of a ship, and as the aurora brightened and faded so did the sound which accompanied it.” If under these conditions the sound was really due to the aurora, the latter, as Captain Dawson himself remarks, must have been pretty close.
25. Usually the electric potential near the ground is positive compared to the earth and increases with the height (see Atmospheric Electricity). Several Arctic observers, however, especially Paulsen (18) have observed a diminution of positive potential, or even a change to negative, for which they could suggest no explanation except the presence of a bright aurora. Other Arctic observers have failed to find any trace of this phenomenon. If it exists, it is presumably confined to cases when the auroral discharge comes unusually low.
26. Artificial Phenomena resembling Aurora.—At Sodankylä, the station occupied by the Finnish Arctic Expedition of 1882–1883, Selim Lemström and Biese (23) described and gave drawings of optical phenomena which they believed to be artificially produced aurora. A number of metallic points, supported on insulators, were connected by wires enclosing several hundred square metres on the top of a hill. Sometimes a Holtz machine was employed, but even without it illumination resembling aurora was seen on several occasions, extending apparently to a considerable height. In the laboratory, Kr. Birkeland (19) has produced phenomena bearing a striking resemblance to several forms of aurora. His apparatus consists of a vacuum vessel containing a magnetic sphere—intended to represent the earth—and the phenomena are produced by sending electric discharges through the vessel.
27. Theories.—A great variety of theories have been advanced to account for aurora. All or nearly all the most recent regard it as some form of electrical discharge. Birkeland (19) supposes the ultimate cause to be cathode rays emanating from the sun; C. Nordmann (24) replaces the cathode rays by Hertzian waves; while Svante Arrhenius (25) believes that negatively charged particles are driven through the sun’s atmosphere by the Maxwell-Bartoli repulsion of light and reach the earth’s atmosphere. For the size and density of particles which he considers most likely, Arrhenius calculates the time required to travel from the sun as forty-six hours. By modifying the hypothesis as to the size and density, times appreciably longer or shorter than the above would be obtained. Cathode rays usually have a velocity about a tenth that of light, but in exceptional cases it may approach a third of that of light. Hertzian waves have the velocity of light itself. On either Birkeland’s or Nordmann’s theory, the electric impulse from the sun acts indirectly by creating secondary cathode rays in the earth’s atmosphere, or ionizing it so that discharges due to natural differences of potential are immensely facilitated. The ionized condition must be supposed to last to a greater or less extent for a good many hours to account for aurora being seen throughout the whole night. The fact that at most places the morning shows a marked decay of auroral frequency and intensity as compared to the evening, the maximum preceding midnight by several hours, is certainly favourable to theories which postulate ionization of the atmosphere by some cause or other emanating from the sun.
Authorities.—The following works are numbered according to the references in the text:—(1) A. Angot, Les Aurores polaires (Paris, 1895); (2) H. Fritz, Das Polarlicht (Leipzig, 1881); (3) Svante August Arrhenius, Lehrbuch der kosmischen Physik; (4) Joseph Lovering, “On the Periodicity of the Aurora Borealis,” Mem. American Acad. vol. x. (1868); (5) Sophus Tromholt, Catalog der in Norwegen bis Juni 1878 beobachteten Nordlichter; (6) Observations internationales polaires (1882–1883), Expédition Danoise, tome i. “Aurores boréales”; (7) Carlheim-Gyllensköld, “Aurores boréales” in Observations faites au Cap Thorsden Spitzberg par l’expédition suédoise, tome ii. 1; (8) “Die Österreichische Polar Station Jan Mayen” in Die Internationale Polarforschung, 1882–1883, Bd. ii. Abth. 1; (9) Henryk Arctowski, “Aurores australes” in Expédition antarctique belge . . . Voyage du S. Y. “Belgica”; (10) G. C. Amdrup, Observations . . . faites par l’expédition danoise; H. Ravn, Observations de l’aurore boréale de Tasiusak; (11) K. Sven. Vet.-Akad. Hand. Bd. 31, Nos. 2, 3, &c.; (12) Sitz. d. k. Akad. d. Wiss. (Vienna), Math. Naturw. Classe, Bd. xcvii. Abth. iia, 1888; (13) Proc. Roy. Soc., 1906, lxxvii. A, 141; (14) Kongl. Sven. Vet.-Akad. Hand. Bd. 15, No. 5, Bd. 18, No. 1; (15) Bull. Acad. Roy. Danoise, 1889, p. 67; (16) Voyages . . . pendant les années 1838, 1839 et 1840 sur . . . la Recherche, “Aurores boréales,” by MM. Lottin, Bravais, &c.; (17) Missions scientifiques . . . au Spitzberg . . . en 1899–1902, Mission suédoise, tome ii. VIIIᵉ Section, C. “Aurores boréales”; (18) Bull. Acad. R. des Sciences de Danemark, 1894, p. 148; (19) Kr. Birkeland, Expédition norvégienne 1899–1900 pour l’étude des aurores boréales (Christiania, 1901); (20) Terrestrial Magnetism, vol. iii. (1898), pp. 5, 53, 149; (21) Astrophysical Journal, 1904, xix. p. 187; (22) Rapports présentés au Congrès International de Physique réuni à Paris, 1900, iii. 438; (23) Expédition polaire finlandaise (1882–1884), tome iii.; (24) Charles Nordmann, Thèses présentées à la Faculté des Sciences de Paris (1903); (25) Terrestrial Magnetism, vol. 10, 1905, p. 1; (26) Observations of the International Polar Expeditions 1882–1883 Fort Rae . . . by Capt. H. P. Dawson, R. A. (C. Ch.)
AURUNCI, the name given by the Romans to a tribe which in historical times occupied only a strip of coast on either side of the Mons Massicus between the Volturnus and the Liris, although it must at an earlier period have extended over a considerably wider area. Their own name for themselves in
