In the majority of records of auroral activity the total intensity has been estimated visually and the results expressed in international coefficients ]see 1 of ”References“ at end of paper[. This is a useful method but is liable to inaccuracies due to individual differences in observers and to the difficulty of forming any precise estimate of intensity-variations when these are accompanied by changes in color and by a movement of the luminous regions. In a few cases objective records of activity have been made. The French Polar-Year Party at Scoresby Sound used an electrical device for measuring intensity . It appeared to be very satisfactory, even detecting auroras during foggy weather. The records show that the luminous intensity may vary in the ratio 1:1000. The method does not seem, however, to be in general use. Continuous photographic recording of the intensity of various spectral lines has also been used . The plates were exposed for a constant interval and the developed image-densities compared. In this way a curve of average activity during successive intervals of time can be plotted.
L. B. Snoody, V. C. Snoody
Eos, Transactions American Geophysical Union