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

IAGA endorsed

IAGA Bulletin 37, 1975, 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Historical references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 (554.2KB)

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