CHAPTER XV.
THE AURORA.
1. The Northern lights had shone for these two winters with incomparable splendour, not, indeed, with the quiet diverging beams, sometimes observed in our northern latitudes, and different also from the phenomena which have been seen and noted in recent years, even in Central Europe; they resembled rather those we saw in East Greenland, save that the brilliancy and intensity of their colours were far greater.
2. It is very difficult to characterize the forms of this phenomenon, not only because they are manifold, but because they are constantly changing. Sometimes the Aurora appears like flaming arches with glowing balls of light; sometimes in irregular meridians painted on the heavens, sometimes in brilliant bands and patches of light on the sky. Each of these forms was frequently developed from a different one, but towards morning the last-named appearance was the most general.
3. The movement of the waves of light gave the impression that they were the sport of winds, and their sudden and rapid rise resembled the uprising of whirling vapours, such as the Geysers might send forth, which generally assumed the form of enormous flames, except that they were transparent and mist-like. In many cases the Aurora much resembled a flash of summer lightning conceived as permanent. It appeared almost always in the south, and was visible from September till March, during which period it was to us the only external excitement which we had. The illuminating power of the Aurora, when its colours were most brilliant and intense, was inferior to the illuminating power of the full moon. Some rare cases excepted, this was either so small or so transitory, that it had no influence on the darkness of our long winter nights. Like a stream, or in brilliant convolutions, the light rushed over the firmament, as well from east to west as from west to east. The formation of the corona (or the convergence of the streamers in the direction of the inclination needle) was sudden, and short in its duration, and frequently happened more than once in the course of a night. Its greatest intensity was from eight till ten o’clock at night. It was never accompanied with sound.[26] The sketch we have given represents one of its most characteristic forms. The inner parts of the flames are usually whitish green, and their edge on the upper side red, on the lower green.
4. Brilliant auroras were generally succeeded by bad weather. Those on the other hand which did not rise to any great height in the sky, or which did not show any special mobility, were regarded as the precursors of calms. None of the theories which have been ventilated are in exact accordance with all the manifestations of these northern lights. The undulating motion of their waves of light, their rolling forth like pillars of smoke driven by winds, has hitherto remained unexplained. Although electrical processes, still unknown, seem to be the main causes of the Aurora, atmospheric vapours may, however, have a considerable part in producing the phenomenon; and nothing so much favours this supposition as the indefinite form in which it often appears. Its occurrence during the day, i.e. light clouds with its characteristic movement, has been rather imagined than actually observed. The transition of white clouds into auroral forms at night has never at least been satisfactorily proved. Falling stars pass through the northern lights without producing any perceptible effect, or undergoing any change. A dirty sulphur yellow was characteristic of all auroras when the sky was overcast with mists or when they were seen by moonlight. In clear weather they were colourless.
THE AURORA DURING THE ICE PRESSURE.
5. Their influence on the magnetic needle was very variable. While the quiescent and regular arches had little or no effect, the quicker and more fitful streamers, especially when accompanied with prismatic colours, produced great disturbance in it. Sir John Ross remarked, that the aurora when tinged with deep red colour had a great effect on it, although he completely stultifies his observation by his supposition, that the phenomenon was produced by rays of the sun reflected on the vast fields of snow and ice surrounding the Pole. Parry in 1820 could discover no effect from it either on the magnetic needle or on the electrometer. During the winter of 1872-3, the character of the northern lights was much altered, though their colour remained constant. At first they consisted chiefly of bands of light, running from the south-northwards. At a later period of that winter they assumed for the most part the appearance of coronæ, and then their direction was from the north southwards. During the voyage of the Tegetthoff the observations of the behaviour of these lights and of the magnetic constants were taken by Weyprecht, Brosch, and Orel by means of a magnetic theodolite, a dipping needle, and three variation instruments. The extraordinary disturbances of the needle rendered the determination of exact mean values for the magnetic constants impossible. The diminution of their intensity was considerable during the continuance of auroras. In 79° 51′ N. Lat. and 58° 56′ E. Long. the declination amounted to 19½° E. and the inclination to 82° 22′. The ice-pressures which occurred in December, 1873 together with the tedious preliminaries in fixing the magnetic instruments, prevented these officers from carrying out their labours regularly till the next month. The following are the principal results of these observations: (1) The magnetic disturbances were of extraordinary magnitude and frequency. (2) They were closely connected with the aurora; and they were greater as the motion of the rays was more rapid and fitful, and the prismatic colours more intense. Quiescent and regular arches, without changing rays or streamers, exercise almost no influence on the needle. (3) In all the disturbances the declination needle moved towards the east, and the horizontal intensity decreased while the inclination increased.