The selection of the quietest days (Q-days) and most disturbed days (D-days)
of each month is deduced from the Kp
indices on the basis of three criteria for each day:
The sum of the eight Kp values.
The sum of squares of the eight Kp values.
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
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 greater
than 6, or marked by the letter K if Ap
less or equal 6 and either one Kp value greater
than 3 or two Kp values greater than 2+.
A selected disturbed day is considered not really disturbed
and marked by an asterisk (*) if Ap less than 20.