# Geomagnetic ap, Ap, Cp, and C9 Indices

## Geomagnetic ap Index

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.

## Geomagnetic Ap Index

The daily index *Ap* is obtained by averaging the eight values of *ap* for each day.

## Geomagnetic Cp Index

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:

## Geomagnetic C9 Index

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: