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Matzka et al. (2021) describe the state of the Kp index 70 years after its introduction and serve as a reference for users of the IAGA-endorsed Kp index as well as for the derived indices ap, Ap, Cp and C9 in their definitive and nowcast version as well as for the International Quiet and Disturbed Days. This dataset is provided by GFZ and derived from indices or near real-time geomagnetic observatory data provided by 13 contributing observatories. Please cite this data publication as well as the accompanying publication (Matzka et al., 2021) when using the Kp index or any of the derived indices obtained from this dataset or from its copies provided by GFZ or other institutions.

The Kp index
The Kp index was introduced by Bartels (1949). We follow here generally the notation of Bartels (1957). A comprehensive list of references on the Kp index is provided in Matzka et al. (2021), which also describes the near real-time distribution of the indices.

DOI and versioning
With the introduction of the DOI ‘https://doi.org/10.5880/Kp.0001’ for the dataset, the DOI landing page and the associated FTP server linked to this DOI become the dataset's primary archive (while the other established index distribution mechanisms at GFZ will be maintained in parallel). With the DOI, the dataset can grow with time, but a change of the data, once published, is not possible. If necessity arises in the future to correct already published values, then the corrected dataset will be published with a new DOI. Older DOIs and data sets will then still be available. For each DOI, an additional versioning mechanism will be available to document changes to the files such as format changes, which do not affect the integrity of the data. The DOI 10.5880/Kp.0001 identifies the current version.