Diurnal variation of horizontal magnetic force at Kodaikanal

Abstract

The diurnal variation of horizontal force at Kodaikanal on each of the international quiet days during the summer solstices of the years 1949-1951 is subjected to harmonic analysis. The harmonic coefficients of the same period are plotted on a harmonic dial. In the case of a strictly persistent periodicity, affected only by random errors, the probable error figure will be a circle. For the 24-hourly wave the probable error figure is found to be an ellipse having the ratio of the axes, 1.80. The direction of the major axis indicates that the amplitude varies much more than the phase. An attempt is made to find out whether this large variation in amplitude can be accounted for by regular causes like degree of disturbance, variation of sunspot number etc. Sunspot number accounts only to a small extent for this large variation of the amplitude. The variability is most fortuitous. It is also noticed that as the value of the first harmonic coefficient decreases, the corresponding decrease in the second harmonic coefficient is less and the second component may even be larger than the first when the latter is very small.