Daily Variations in the Intensity of the Auroral Green Light from the Night Sky
Koiwai, M.
Abstract
The observations of Lord Rayleigh, McLennan, Slipher, Dufay, Karandikar & etc. have proved beyond doubt the occurence of the daily variaiotns in the intensity of the auroral green light from the night sky.
The author has recently constructed a primitive filter camera in order to observe the daily variaiotns in the intensity of the auroral green light in Japan.
The arrangement (Plate I) was made so that a slit could be slowly moved over a photographic plate that was held close up to it. The motion was provided by the fine wire which was connecting with the axis of a clock. The width of the slit was 0.5cm, and the axis of the clock was controlled so that each point illuminated on the photographic plate was exposed for about 1.5 hours.
The filter used was a kind of the "aurorak filters", one of which was a thick didymium glass and the other was a thin yellow-green filter.
The photograph reproduced in (Plate II) shows the results of the testing of this auroral filter by using a electric lamp and a neon. the transmission was practically limited to a range of 400 A., between λ5400 and λ5800. As the auroral green line is comparatively more stronger in intensity than the continuous spectrum about that region, the filter is suited for observations on the night sky light corresponding to the auroral green line by using a orthochromatic plate.
The exposures were made with the photographic plate in a horizontal position, so that it was directly exposed to about a hundred square degrees of the sky. Between November 18 and December 29, eleven exposures were made and most of them have indicated variations in the intensity of the green light from the sky of the night.
These variations were classified in two types, one of which was the gradual increase in the intensity towards the midnight and had a maximum at half an hour after midnight (Plate III), and the other type of variations was on the contrary the gradual decrease in the intensity towards the midnight and gave a minimum at the midnight (Plate IV).
The author denominates the former W-type and the latter V-type.
Rayleigh, McLennan and others have observed only the W-type and Karandikar & Ramanathan in India have observed the V-type.
It will be interesting to see if any connection can be established between the daily variations known to occur in the state of ionisation of the upper atmosphere and the variations indicated here in the intensity of the night sky light.
The variations of the electron-density of the F2-layer show the secular and seasonal variations, both of which are very similar to the intensity variations of the night sky light.
Now the diurnal variations of the electron-density of the F2-layer are generally known to have two types: winter type and summer one. The author alludes to the coincidence of both the W-type variation of the night sky light and the winter type of the variation of the electron-density in the F2-layer, and also the V-type variation and summer type.
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