. Compendium of meteorology. Meteorology. FiG. 4.âAurorae situated in the border region between the dark and sunlit atmosphere. The horizontal line indicates the shadow line. (After Stormer [13].) ing over southern Scandinavia at a considerable dis- tance from the auroral zone, may attain heights up to 800-1000 km. Aurorae appearing at such great heights are always situated in the sunlit atmosphere and, at these low latitudes, appear only during great magnetic storms. Figure 4 shows how the lower limits concen- trate along the shadow line between the dark and the sunlit atmosphere. Spectrograp
. Compendium of meteorology. Meteorology. FiG. 4.âAurorae situated in the border region between the dark and sunlit atmosphere. The horizontal line indicates the shadow line. (After Stormer [13].) ing over southern Scandinavia at a considerable dis- tance from the auroral zone, may attain heights up to 800-1000 km. Aurorae appearing at such great heights are always situated in the sunlit atmosphere and, at these low latitudes, appear only during great magnetic storms. Figure 4 shows how the lower limits concen- trate along the shadow line between the dark and the sunlit atmosphere. Spectrographic observations by Stormer have shown that the nitrogen band in sun- lit aurorae is strongly enhanced relative to the green auroral line. Spectrum of the Aurorae The intensity of the auroral luminositj^ is low, and if spectrographs are to be used, they must be of con- siderable light-gathering power. The spectrum con- sists of a number of lines and bands; more than one hundred are listed in the wave-length tables. They extend from 8100 A (infrared) to 3100 A (ultraviolet) where ozone absorption cuts off the spectrum. Wave Lengths and Average Intensities of Spectral Lines. By exposing a spectrum over a number of nights, an average spectrum with intensities approximating the values given in Table I is obtained. Here only the stronger of the auroral lines, together with their identifi- cations, are listed. The spectnim is dominated by the nitrogen band systems, the negative group (N G) and the first and second positive groups (1 P (? and 2 P G), which are all well known from the study of gas dis- charges. The visible color of the aurorae, however, is produced by the atomic oxygen lines 5577 A in the green (Fig. 5) and the doublet 6363, 6300 A in the red. In addition to the stronger lines a considerable number of weak lines and bands have been listed, some of which seem to be atomic lines of 0 and N. Of special interest is the appearance of the forbidden nitrogen line Ni {'S - ^P
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