Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

QUIETEST CK-DAYS

Available from 1868 onwards
 
Type of event quietest days over 24 or 48 hours
 
Purpose To define periods of magnetic quietness with regard to the irregular geomagnetic activity over 24 hours or 48 hours centered on the considered UT day.
 
Network based on the aa network made of 2 antipodal magnetic observatories.
(see list of actual and previous aa magnetic observatories)
 
Derivation "Really Quiet (C)" and "Quiet (K)" days, over 24 hours (CK24) or 48 hours (CK48), are deduced from the aa indices on the basis of two criteria for each UT day:
   1. The mean of the aa values.
   2. The sum of the aa values transformed in p weigth according to the
       following array:
Ranges (nT) p
aa ≤ 17 0
17 < aa ≤ 21 1
21 < aa ≤ 28 2
28 < aa ≤ 32 4
32 < aa 6
 

Quietest days over 24-hours (CK24) are UT days with mean(aa) = Aa < 13 nT;
Two categories are defined:
   - Really quiet days are designed by letter "C" with ∑p < 4
   - Quiet days are designed by letter "K" with ∑p ≥ 4

Quietest days over 48-hours (CK48) are defined by 48 hours period centered onto the considered UT day with mean(aa) < 13 nT;
Two categories are defined:
   - Really quiet days are designed by letter "C" with ∑p < 6
   - Quiet days are designed by letter "K" with ∑p ≥ 6
 
ISGI Collaborating Institute EOST, Strasbourg, France
(previously LATMOS, Guyancourt, France)
 
IAGA endorsed IAGA Bulletin 37, 195, p. 128, resolution 3
 
Historical references • Mayaud, P. N. (1971) - Une mesure planétaire d'activité magnétique basée sur deux observatoires antipodaux, Ann. Geophys., 27, 67-70.
• Mayaud, P. N. (1972) - The aa indices: A 100-year series characterizing the magnetic activity. J. Geophys. Res., 77(34), 6870-6874. Get PDF (555K)
 
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