摘要：The K index is one of the oldest universal indices of geomagnetic activity, introduced in 1938 by Julius Bartels, that is still being widely used. Up to the present day, long-term time series of homogeneous K index records have been accumulated at data repositories all over the world. The multidecadal practice of its application makes it an indispensable source of information for the retrospective analysis of solar–terrestrial interaction for nearly eight solar cycles. Most significantly, while studying the historical geomagnetic data, K index data sheets are in certain cases far easier for automated analysis than the conventional analogue magnetograms. 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 the 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 the retrospective analysis and study of geomagnetic events in the past, as well as for data validation or forecasting (Sergeyeva et al., 2020). The dataset is available at: https://doi.org/10.1594/PANGAEA.922233, last access: 16 September 2020.
