From K to Ks
There is a pronounced diurnal variation in the K index at a single station, with intervals close to local midnight being substantially more disturbed in comparison with those centered on local noon. This diurnal variation also exhibits seasonal and latitudinal dependence. Mere averaging of the logarithm of activity was therefore not considered proper unless the stations were densely and evenly distributed over the world. To overcome this, a standardization procedure was developed by J. Bartels for assigning to each K value a standardized alternate - designated by Ks - which could be more refined than K in the sense that it is given in thirds.
The present conversion tables for three seasons have been prepared using selected data from 1943 to 1948 (except Niemegk). The three seasons are:
The months around winter solstice: January, February, November and December.
The months around the equinoxes: March, April, September and October.
The months around summer solstice: May, June, July and August.