A study on the K-index at Kakioka and Memambetsu
Uesugi, T., Iwase, I., Koike, K. & Yoshida, A.
We investigated the difference between the K-index values at the Kakioka and Memambetsu Magnetic Observatories during the period of 45 years from 1958 through 2002. We show that there appeared a tendency that the K-index value at Memambetsu was determined to be larger than that at Kakioka after revision of the K-index scale in March 1978. Then we compared the frequency distribution of the K-index at Memambetsu with that of Kp to show that the number of large K-index values at Memambetsu is conspicuously less than that of Kp, and the frequency distribution of the K-index values for the same Kp and that of the Kp values for the same K-index at Memambetsu are remarkably non-symmetric. We think there is something different between the type of geomagnetic disturbances for which the K-index at Memambetsu is the same as Kp, and the type for which the K-index at Memambetsu is notably smaller than the Kp value. It is further shown that a correlation is recognized! between changes in the number of K-index values of 6 or larger and the Wolf number of solar spots. This can be understood if we note that a large K-index is likely to be observed during large storms, most of which are storms with sudden commencement, and that there is a correlation between changes in the Wolf number and the number of storms with sudden commencement (Yoshida et al., 2004). On the other hand, the tendency that periodic magnetic disturbances are apt to be observed in the declining stage of solar activity (Koike, 1991) can be understood if we pay attention to the features that periodic activities are associated with storms with gradual commencement caused by high-speed solar wind from a coronal hole, and that storms of that kind become dominant in the declining stage of solar activity.