Full Text
From Current Literature
The Nature of the Northern Lights
(J. Stark. Über die Natur der Nordlichtstrahlen. “Die Naturwissenschaften” 6, p. 145, March 1918).
The northern lights are located around the magnetic poles of the earth and are closely connected with solar activity (sunspots). These facts make it possible to conclude with considerable probability that the peculiar glow is caused by a stream of electrically charged particles sent out by the sun and bombarding the upper layers of the atmosphere. Two assumptions are possible as to the nature of these particles: either they are electrons, or they are positively charged ions. The first hypothesis was chiefly advanced by Birkeland1, the second has recently been put forward by Vegard2. Stark resolves this dispute very convincingly in favor of the second hypothesis.
For many elements one has to distinguish, as was established by Stark and his pupils, between the “spark” and the “arc” spectrum; moreover, the difference is observed both in the positions and number of the lines, and in the ratio of the intensities of individual lines. “Spark” spectra can be obtained when fast cathode and canal rays pass through a given gas; “arc” spectra are characteristic only of canal (positive) rays. From determinations of the height of the northern lights one may conclude that they arise in those layers of the atmosphere whose principal constituent is nitrogen, as is also confirmed by the spectral analysis of the northern lights. However, alongside the known lines of nitrogen, a number of new lines have been found in the spectrum of the northern lights, including the bright yellow-green line hitherto attributed to an element unknown on earth, “geocoronium.” Until quite recently it had not been possible to obtain the “arc” spectrum of nitrogen, and only Stark, together with his pupils Hermann and Hardtke, using the method of canal rays in an atmosphere of nitrogen, finally solved this problem. Most of the newly found lines coincided exactly with the characteristic lines of the northern lights, including the famous “geocoronium” line. The coincidence is observed not only in the positions of the lines, but also in the distribution of intensities. Thus the puzzle of the spectrum of the northern lights is explained, and the exciters of this kind of spectrum can only be positive ions emitted by the sun.
Many investigators have pointed out that quite often in the spectrum of the northern lights a line has been observed near the blue hydrogen line 486 μμ,
whereby this line sometimes coincides with the hydrogen line, and sometimes is shifted. Stark attributes the appearance of this line to the luminescence of the ion itself, in the present case the hydrogen ion: the displacement of the lines serves as a clear indication (according to Doppler’s principle) of the great velocity of this ion. Unfortunately, the spectrograms of the aurorae are so faint that it is not yet possible to make an exact determination of the position of the lines, and consequently of the velocities. The change in the color of the aurorae must, apparently, be ascribed to the different nature and different velocities of the exciting positive ions.
S. Vavilov.