**Definition:** An individual *K* index is an integer in the range 0 to 9 corresponding to a
class that contains, after eliminating the regular daily variation (*S _{R}*),
the largest range of geomagnetic disturbances (a

The limits of these classes at a particular observatory are defined with the intent of producing a geomagnetic disturbance characterisation that does not depend significantly on the location of a sub-auroral, mid- or low- latitude observatory.

**Limits of class:** The limits of classes at a particular observatory are defined with the intent of producing
a geomagnetic disturbance characterisation that does not depend significantly on the location of a sub-auroral, mid- or low- latitude
observatory.

The limits may vary from one observatory to another
since they depend on the corrected geomagnetic latitude.

ISGI-headquarters are in charge of the computation of *L9* value for each magnetic observatory.

**Computation of K indices:**
The original definition of

Different algorithms enabling computer derivation of K indices
were then developed and carefully assessed in the frame of an
international comparison organised by the IAGA Working Group “Geomagnetic indices” (Coles & Menvielle, 1991;
Menvielle, 1991; Menvielle et al., 1995).Basically, these algorithms estimate the S variation from the magnetograms. The geomagnetic disturbances, from
which the _{R}K indices are computed, are derived as the difference between the observed variations and the computer
estimated S ones.
_{R} Example: S
_{R} and K determination by the FMI algorithm (Sucksdorff et al., 1991) for a
moderately disturbed day (2010, April 12th; Am = 30 nT) at the PAF observatory. |