Noncyclic variation andS _q

Summary

A method is given of computing estimates of noncyclic variation and Fourier coefficients of the geomagnetic transient daily variationS _q corrected for noncyclic variation.

The method may be applied directly to hourly mean values as they occur printed in Magnetic Observatory Yearbooks without the need for preliminary or separate computations to determine the noncyclic variation. As the method uses all 24 hourly mean values, noncyclic variation can be determined for Greenwich International quiet days, at any observatory, and the effect of short period disturbances and day-to-day variability ofS _q on estimates of noncyclic variation is minimized, and no spurious longitude effect is introduced.

If only the noncylic variation is required an expression is given using the Fourier coefficients, the first moment and sum of the hourly values for the evaluation of an estimate with the advantages outlined above.

The determination of confidence limits is uncomplicated, thus results may be accurately weighted if used in further analyses, spherical harmonic analysis, for example.