Fig. 1
Cumulative frequency distributions of the H-range of geomagnetic storms at a Kakioka (Feb. 1923 to Dec. 2013; n = 1849), b Memambetsu (Feb. 1923 to Dec. 2013; n = 1150), and c Kanoya (Feb. 1923 to Dec. 2013; n = 1096). The straight line shows the estimated power law relation of the distribution. Intercepts of functions are rough estimates. The vertical axes show the cumulative number of occurrences and the horizontal axes show the event magnitude, indicated by the log 10 of the H-range of the storm. H-range is defined as the difference between the maximum and minimum values of the horizontal (H) component of the geomagnetic field during the storm event