(Storm) Sudden Commencements (ssc) are defined by an abrupt increase or decrease in the northward component of the geomagnetic field, which marks the beginning of a geomagnetic storm or an increase in activity lasting at least one hour. The sudden commencements and solar flare effects (sfe) are from magnetograms of the worldwide network of magnetic observatories.
The stations, together with their abbreviations, are given in the series IAGA Bulletin No. 32 which contains the yearly compilation of these data. Before January 1966 these reports were published periodically in Journal of Geophysical Research. From then until 1970 they were published quarterly in Solar-Geophysical Data (SGD).
Beginning with December 1970, these data are published monthly and, thus, are based on fewer reports and differ slightly from similar data published previously. The decision to publish this less complete report was made in order to make the data available more rapidly. Only events reported by at least five observatories are reported.
The latest ssc reports are provided by Observatorio del Ebro, Roquetes, Spain, on behalf of the IAGA Service on Rapid Magnetic Variations.