K indices and K-derived magnetic activity indices: context’s reminder

The K index was devised by Bartels et al. (1939) to provide an objective monitoring of irregular geomagnetic activity at subauroral latitudes. K indices are based upon geomagnetic disturbances, measured in horizontal geomagnetic components at magnetic observatories, after « eliminating » the regular daily variation. An individual K index is an integer in the range 0 to 9 corresponding to a class that contains the largest range of geomagnetic disturbances (in either of the two horizontal components) during a 3-hour UT interval. Limits of range vary from one observatory to another since they depend on the corrected geomagnetic latitude of the observatory.

A great number of Space Weather applications rely on K-derived magnetic activity indices at subauroral latitudes. These historical indices; endorsed by IAGA such as Kp, aa and am; represent unprecedented homogeneous time series, up to more than 150 years, highly valuable for all studies related to long-term geomagnetic activity.

However, one has to keep in mind that local K indices and subauroral related ones (K-derived) were developed during other time, under specific societal and technological conditions.

We recall the local K indices derivation processes and characteristics to enlight possible nowadays drawbacks and their simple mitigations.