The presented collection includes the results of the K index determination at 41 geomagnetic observatories of the former USSR for the period from July 1957 to early 1990s. This unique collection was formed at the World Data Center for Solar-Terrestrial Physics in Moscow. The historical data, which are offered to the international scientific community, cover the second half of the 20th century and can be used for retrospective analysis and study of geomagnetic events in the past as well as for data validation or forecasting.

K index is a quasi-logarithmic three-hour index characterizing the change in geomagnetic activity at a specific observatory in the three-hour time intervals, starting at 00 h of universal time (UT), and is expressed in balls. K index takes values from 0 (quiet magnetic field) to 9. The value of K = 9 corresponds to a very strong geomagnetic disturbance. Taking into account that the amplitude of the geomagnetic disturbance depends on the latitude of the place of observation, the value of K = 9 corresponds to a perturbation greater than 2500 γ in the auroral zone and greater than 300 γ at low latitudes (excluding the equator). For all other observatories located in the polar cap and mid-latitudes, a value of K = 9 corresponds to perturbations of less than 2500 γ, but greater than 300 γ.Each observatory, publishing data on the K index, necessarily indicates a lower limit for K = 9.