On the auroral spectrograms taken at Chesterfield, Canada, during 1932–33

Abstract

This is a study of 693 spectrograms of the auroral and the night‐sky light that were taken at Chesterfield during the International Polar Year. The spectrograms had been obtained with successive exposures of about 1.5 hours throughout each night. In addition to presenting spectrographic pictures of the variations in auroral activity throughout each night they showed the effect of twilight and of moonlight on the auroral radiations. The radiations that appeared on the spectrograms are identified with reasonable certainty, and their frequencies of occurrence are computed. Evidence is presented to show that the radiations from the upper atmosphere at Chesterfield are characteristically auroral at all times, and that the green‐line luminescence of the night‐sky at lower latitudes does not occur continuously at Chesterfield. A comparison is made between the frequencies of occurrence and the intensities of the two red‐radiations on moonlight nights and on dark nights to show the effect of moonlight on these radiations. Similar effects are shown to occur during twilight‐periods. The possibility of using spectrographic observations to deduce auroral character‐numbers is indicated. The mean nightly variation in the intensity of the aurora for 30 clear nights is deduced from the apparent changes in the intensity of 5577Å, of 3914Å, and of all the radiations appearing on the spectrograms, and is compared with the mean nightly variation as deduced from the visual observations. The appearance of the radiation 3578Å (as an indicator of bright, active aurora), is used to find the mean nightly variation in auroral activity during the winter. The mean nightly variation in auroral activity is compared with the mean diurnal ranges of the earth‐potential and of the magnetic declination used as indicators of earth‐current and magnetic activity, respectively.