The three-hour index ap and the daily indices Ap, Cp and C9 are directly related to the Kp index. In order to obtain a linear scale from Kp, J. Bartels gave the following table to derive a three-hour equivalent range, named ap index:
This table is made in such a way that at a station at about dipole latitude 50 degrees, ap may be regarded as the range of the most disturbed of the two horizontal field components, expressed in the unit of 2nT.
The daily index Ap is obtained by averaging the eight values of ap for each day.
In order to replace the somewhat subjective index Ci, the Cp index - the planetary daily character figure - was developed. Cp is a qualitative estimate of overall level of magnetic activity for the day determined from the daily sum of eight ap amplitudes. Cp ranges, in steps of one-tenth, from 0 (quiet) to 2.5 (disturbed) and is defined according to the following table:
Another index devised to express geomagnetic activity on the basis of the Cp index is the C9 index. It converts the 0 to 2.5 range of Cp to one digit between 0 and 9 and is defined by the following conversion table: