Long Time Variations of the Attenuation Constant and the Zero-Level of Dst Inferred from Variations of the Geomagnetic Horizontal Component observed at Kakioka
Yanagihara, K.
Abstract
The geomagnetic horizontal component H increases after a disturbance to the next, reflecting a decline of Dst-field. The quiet day occurs in the course of the H-increase. Daily mean values of H for a successive occurrence of the quiet day shows the increase is exponential, suggesting a possibility to find the final level of the H-increase, that is the zero level of Dst. However the analysis to find the final level from a single H-increase is not easy because of noises inc1uded in the daily mean value of H, for example day-to-day changes of the diurnal variation, solar wind compression, minor disturbance and the base line value of the variometer. Some statistical treatments are necessary.
Quiet days are newly selected by a common criterion for disturbance activity instead of that of the international calm day which is not equal for each month. The present analysis uses average H variations observed at Kakioka for the cases that the quiet day occurs successively for 4 days. The result shows an interesting long time variation of the attenuation constant of the H variation. After the middle of 1950’s the attenuation constant increases linearly compared with the invariability before the epoch. A magnetospheric pollution due to human activity might be a cause of the increase. The zero level of Dst is higher by 10-30 nT than the quiet day mean. The long time variation of the difference of about 20 nT inc1udes two components originating in the variations of the attenuation constant and the intensity of the disturbance. Excluding the two components the residual shows a ’60-year period variation' of 10 nT. The cause of the 60-year period variation remains to be studied together with the cause of the increase o( the attenuation constant after 1950’s.
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