Service International des Indices Géomagnétiques
International Service of Geomagnetic Indices

International Q-days and D-days

Available from 1932 onwards
Type of event international quietest (Q-Days) and most disturbed days (D-Days)
Purpose To define periods of magnetic quietness from a ranking of the UT days of each month with regard to the irregular geomagnetic activity. The most popular outcomes are the lists of the 10 quietest and 5 most disturbed days of the month.
Data interval 10 Q-Days and 5 D-Days per month
Network based on the Kp network.
(see list of actual and previous Kp magnetic observatories)
Derivation International most quiet and most disturbed days of each month are deduced from the Kp indices on the basis of three criteria for each day:
   1. The sum of the eight Kp values.
   2. The sum of squares of the eight Kp values.
   3. The maximum of the eight Kp values.

According to each of these criteria, a relative order number is assigned to each day of the month, the three order numbers are averaged and the days with the lowest and the highest mean order numbers are selected as the five (respectively ten) quietest and the five most disturbed days.

It should be noted that these selection criteria give only a relative indication of the character of the selected days with respect to the other days of the same month. As the general disturbance level may be quite different for different years and also for different months of the same year, the selected quietest days of a month may sometimes be rather disturbed or vice versa.
In order to indicate such a situation, selected days which do not satisfy certain absolute criteria are marked as follows.
   • A selected quiet day is considered "not really quiet" and is marked by the letter A if Ap > 6, or marked by the letter K if Ap ≤ 6 and either one Kp value > 3 or two Kp values > 2+.
   • A selected disturbed day is considered "not really disturbed" and marked by an asterisk (*) if Ap < 20.

More specific and detailed information may be found on the GFZ website devoted to Kp and related geomagnetic indices.
ISGI Collaborating Institute GFZ Potsdam, Germany
IAGA endorsed introduced in 1905 by the International Commission of Terrestrial Magnetism and Atmospheric Electricity. The present classifications inherit from the tradition initiated by this decision.
Historical references • Johnston, H.F. (1943)- Mean K-indices from twenty one magnetic observatories and five quiet and five disturbed days for 1942, Terr. Magn,. Atmos. Elec., 47, 219, doi:10.1029/TE048i004p00219.