UK Solar System Data Centre
Copyright WDC C1 for STP at RAL.
For years 1957 onwards, hourly index values have been derived at the WDC-C2 for Geomagnetism, Faculty of Science, Kyoto University by M. Sugiura and T. Kamei together with associates from NASA.
Definitive values are available on average 12-18 months after the event. Prompt data is available after one day, and provisional values issued a couple of months later.