On the Frequency Distribution of Geomagnetic K Indices in the Northwestern Pacific Region Over the 19–24 Solar Cycles

K index is the oldest measure of geomagnetic activity estimation in a unified scale over the globe, maintained until today. Recently, Russian and Japanese scientific teams have managed to digitize a huge collection of K index analog records from the observatories located in the northwestern Pacific region. This has made it possible to study long-term evolution of geomagnetic activity in this area over 1954–2020. Using these unique data, we reveal their most appropriate distribution laws. We also analyze the correlation between their time-varying distribution features and sunspot numbers over the 19–24 solar cycles. The distribution features are considered in terms of (a) quasi-linear regression coefficients derived from logarithmic scale cumulative frequency distributions, and (b) occurrences of different K index values binned by year as a function of time. We establish that the probability of K ≥ 8 event detected simultaneously at all observatories in the region over the period of simultaneous data availability is less than one hundredth of a percent; the same for the clusters of neighboring observatories is an order of magnitude larger.