Solar cycle variations in the annual mean values of the geomagnetic components of observatory data.

YUKUTAKE Takesi, CAIN Joseph C.

doi:10.5636/jgg.39.19

Solar cycle variations derived from annual mean values of all day data at observatories contain the effects of magnetic variations such as magnetic storms and solar diurnal changes with periods shorter than one year. The effects of the shorter period variations are, however, usually assumed to be averaged out, and the annual means are widely used for the analysis of the long term variations in the geomagnetic field. The effects of such shorter period variations are examined by comparing three kinds of annual means: the all day mean, the calm day mean and the night-time mean.<br>The difference between the all day annual mean and the calm day annual mean, which is supposed to be caused by an accumulated effect of magnetic disturbances, has been found to vary almost in parallel with the solar cycle variations derived from the all day annual mean itself, exhibiting two peaks during a solar cycle. About 40% of the solar cycle variations in the all day annual mean values is due to this effect. Similarly the difference between the calm day annual mean and the night-time annual mean also shows solar cycle variations, but with a single peak for a solar cycle. The variations in this difference are interpreted to be caused by amplitude variations in the solar diurnal change.<br>The effect of magnetic disturbances and that of diurnal changes contaminate the all day annual mean in different ways. This makes it difficult to extract the solar cycle variations in the geomagnetic field undisturbed by shorter period variations.

Solar cycle variations in the annual mean values of the geomagnetic components of observatory data.

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