Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

Sudden Commencements (SC)

Available from 1869 onwards
Type of event Sudden commencement followed by a magnetic storm/an increase in activity (SSC - Storm Sudden Commencement) or by no further disturbance (SI - Sudden Impulse)
Purpose To provide reference list of events related to magnetosphere compression caused by a sharp increase in the solar wind dynamic pressure.
Network 5 principal + 5 supplementary observatories
(see list of actual SC magnetic observatories)
Determination A SC is basically an unexpected sharp increase in the Northern (X) magnetic component of a considered observatory. Usually, but not always a SC is the precursor to a geomagnetic storm (defined by, for example, a minimum of the Dst index). It has to be noted that more emphasis is put on the change of rhythm in the magnetic activity than on the amplitude of the possible succeeding magnetic storm.
Preliminary lists of SC are compiled on the basis of the reports made by observatories by morphological inspection of their magnetograms.
Definitive lists of SC are made on the basis of visual inspection of magnetograms from 5 selected low latitude observatories.
ISGI Collaborating Institute Observatori de l'Ebre, Spain
IAGA endorsed IAGA Bulletin 37, 1975, p. 130, resolution 6
Historical references • Curto, J.-J., T. Araki, and L. F. Alberca (2007) - Evolution of the concept of Sudden Storm Commencements and their operative identification, Earth Planets Space, 59, I-XII. Get PDF (114.2KB)
• Mayaud, P.N. (1973) - A hundred year series of geomagnetic data, 1868-1967: indices aa, storm sudden commencements(SSC), 256 p., IUGG Publ. Office, Paris.