Wilson, J. W.

2019-02-15T14:27:20Z

2019-02-15T14:27:20Z

1957

Fifty years ago very little was known about conditions in the earth's atmosphere above a height of a few kms. and such ideas as did exist were mainly speculative. Since then, however, much work has been done, powerful new techniques developed, and a store of information accumulated about the inaccessible region above the maximum height of balloon ascent where no direct measurement has been possible until the recent development of high altitude rockets. Indirect methods have been used and one of the most successful of these has been the spectrographic study of the light emitted by these regions. The most intense source of this is the "northern lights" or aurora polaris. Although this phenomenon has been observed and classified for many years - Seneca, in the first century A.D. gives a description of it.- it was not until iTegard and his collaborators in the second decade of the present century analysed its light spectrographically that it was used to provide qualitative information about the condition of the upper atmosphere. The aurora occurs irregularly and is most frequent in a region known as the Auroral zone centred 23.5° from the poles. In addition to this transient phenomenon .Rayleigh found that there was emitted from the earth's lines of the spectrum. This was not a very satisfactory state of affairs as it would have been of great interest to have examined and compared spectra obtained from different phases of a display. However, it was found that if the size of these prism instruments was increased, very little gain in sensitivity was achieved as the absorption of the large prisms involved and the reflections from the many optical surfaces required effectively reduced the theoretical gain to negligible proportions. Thus the desire to have simultaneously high resolution and high light power was found impossible with prism instruments using traditional optics. Vegard4overcame this in his later work by using low dispersion instruments of maximum power to investigate the variations in intensity of the more prominent lines of the spectrum and larger dispersion instruments over long periods to obtain details of the spectrum.

Those who were investigating the airglow were in an even less fortunate position as the intensity of the source they were dealing with was much less than that of the aurora. When using low dispersion instruments with optics of speeds of nearly f /I, which is at the limit of convention4 design, exposure times for useful plates were of the order of 30 to 60 hours. As with the aurora this meant that all spectra obtained were the integrated effects over these long exposure times. This was not very satisfactory but a practical limit seemed to have been reached in the development of the instruments.

With the invention by Schmidt of a camera system which can have speeds considerably faster that f.I. a new tool was available for further progress. Memel in America. designed a special auroral spectrograph using Schmidt's principle. As the dispersive medium he chose a reflecting grating so as to reduce the loss of light and to give nearly uniform dispersion throughout the range. He kept the optical components down to a minimum, used reflecting surfaces wherever possible, and in- corporated a Schmidt camera of f /0,7. This type of instrument has tw ice the range of a prism instrument and can be used from ultra-violet to infra -red with a combination of very high light power and a uniform dispersion of convenient size. Also using modern "blazed" gratings a high percentage of the light can be concentrated into a single order so increasing the efficiency still further.

A few instruments have been built to this pattern. One of these - the instrument used in this investigation - has been lent to St, Andrews Observatory by the Cambridge Air Force Research Centre, Massachusetts, U.S.A. for use over the International Geophysical Year period 1957 -58.

It was planned that in the first instance the following problems should be examined: (1) Measurement of the wavelengths of the lines and bands in the auroral and night airglow spectra over as wide a range of wavelengths and intensities as possible. (2) Estimation of upper atuosphere temperatures from the profiles of suitable nitrogen bands. (3) Investigation of the enhancement of the sodium D lines at twilight.

http://hdl.handle.net/1842/34479

The University of Edinburgh

Annexe Thesis Digitisation Project 2019 Block 22

Spectrographic studies of the aurora polaris and their airglow

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy