Some Reports on the Making and Testing of a Spectrograph for the Permanent Aurora
Koiwai, M.
Abstract
Since the earliest observation of V.M.Slipher, our knowledge of the permanent aurora(or non-polar aurora) has been greatly advanced. The variations of the intensity of the green line λ5577 which has been investigated by Rayleigh, McLennan, Ramanathan & etc. can be classified in three cases; diurnal, annual and secular variations.
As to the diurnal variation, the auroral green line shows the midnight maximum in temperate latitudes(observed by Lord Rayleigh and McLennan) and the midnight minimum in low latitudes(observed by Ramanathan and Karandikar). The observations in England & South Africa appear to indicate an annual periodicity.
The maximum is in October in England, and in April in South Africa. In addition to the annual variation, the observations in England indicate a progressive increase from year to year. This increase has occurred during the rise from sunspot minimum to sunspot maximum, and is through to indicate a periodicity in the 11-year period of the solar cycle.
The author has recently constructed a small spectrograph of suitably high light-power(camera lens having speed F/1.9) in order to observe the auroral green line's annual and secular variations.
The collimator lens is a photographic F/6.3 one, having a focal length of 17.1cm.
The prism is a flint one, and has a refracting angle of 60°(the minimum deviation 48°31' for λ5578). The camera lens is a F/1.9 photographic lens with a focal length of 5cm.
The dispersion power of this instrument is very small(1130Å/mm between λ5853-λ5401), but it gives the full power of its camera lens. The focus was at first adjusted by the Fraunhofer lines and next by the neon. Various experiments have been made in connection with the slit and the photographic plate and the width of the slit was best when 0.45mm. and the plate Agfa-Isochrom.
Using these, the auroral green line was faintly obtained in three hours, and seven hours' exposure gave good spectrograms.
By sliding the plate holder, some eight exposures are possible, so that the intensities of these eight auroral green lines can be compared each other. But different plates cannot be compared even if the deepest care be paid in developing. In order to get a scale of intensities, the author adopted the spontaneous emitted uranium salt packing in the two vessels(see Fig.5). On the uranium salt, the author prepared two wedges consisting respectively many plates of mica and celluloid of equal thickness arranged in the form of steps(Fig.6,a). Furthermore, on each of these wedges a slit(see Fig.6,b) made of metal was furnished. Thus the intensity scale of the standard light can be put in the plate exposed to the night sky. This standard is now in the course of test and as soon as it brings to perfection, the systematic works will be realized.
