The Aurora of February 4

Abstract

A VIEW of the magnificent aurora of Feb. 4 was much interrupted here by great masses of cloud, which frequently drifted over large tracts of the illuminated sky, and towards 8 o'clock collected and descended in a general downpour of rain. Nevertheless enough of it was seen to produce a very striking impression. It began to tinge the southern sky at a considerable altitude so early in the evening that I thought it must have been the reflection of a crimson sunset; nor was I undeceived till I had been to the other side of the house, where I found the western horizon glowing with amber light, in which was no trace of the expected ruddiness. Red continued throughout to be the prevailing hue, chiefly in great diffused masses, but occasionally broken up into filaments and streamers; there was, however, no absence of sheets and columns of the more usual pale green light. The clouds, chiefly heavy cumuli, assumed a strange aspect; sometimes, when opposite to the crimson illumination, reflecting a dull and sombre red, at others, when projected in front of it and enlightened from the other side by the twilight, or the green aurora, standing out inllurid and ghastly contrast. At one period the northern part of the sky, up to a great altitude, though clear and studded with stars, appeared at first sight almost like a black cloud from its contrast to the greenish white sheet which bordered it abruptly at a considerable height on the west; this again passing into crimson masses in the south, and sending out a whitish stream to meet another from the east, and form, probably, for a few moments, a complete bright ring, somewhat south of the zenith, of which, however, only one half could be seen from the post of observation. The light was so intense that even after it had been a good deal obscured by cloud, a large print might have been read without much difficulty, A miniature spectroscope (one of Browning's) brought out some interesting features. The usual yellowish green auroral line was distinct everywhere, and could be perceived even when the instrument was directed to masses of dense cloud; and as was observed by Birmingham on a former occasion, could be made out in the reflection from any suitable terrestrial object; white paper for example exhibited it very obviously. As shown in the brighter greenish patches in the sky, it remained visible even when the slit was so much contracted that the sodium band of a common fire would have been thinned down almost to its smallest breadth before extinction. Such a diminution of light, however, was fatal to the rest of the spectrum, which was a very remarkable one. With a wider slit a crimson band, bearing a fair amount of contraction, was perceptible in the brighter patches of that hue, with a dark interval between it and the principal green band. On the opposite side of that green band, beyond a second similar dark space, was a considerable extent of greenish or bluish light, quite decided, but so feeble as to leave it undecided whether it was of uniform brightness, or (as I suspected) compounded of contiguous bands; beyond this again was another dark space, leading on to a faintly luminous band, too dim to show colour, but which must have taken its place somewhere in the blue. This band, and the darkness adjacent to it on the less refrangible side, were each about as broad as the intensely vivid yellowish green stripe. Could the light have borne sufficient reduction, we should certainly have had three narrow bright bands in the red, green, and blue, the two latter being wide apart, with either a faint separate continuous spectrum, through part of the interval, or possibly several feeble lines, which the widening of the slit fused into one lengthened area.