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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:
`Kp` 0o 0+ 1- 1o 1+ 2- 2o 2+ 3- 3o 3+ 4- 4o 4+
`ap` 0 2 3 4 5 6 7 9 12 15 18 22 27 32
`Kp` 5- 5o 5+ 6- 6o 6+ 7- 7o 7+ 8- 8o 8+ 9- 9o
`ap` 39 48 56 67 80 94 111 132 154 179 207 236 300 400

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:
sum `ap` `Cp` sum `ap` `Cp` sum `ap` `Cp`
0 ... 22 0.0 121 ... 139 0.9 562 ... 729 1.8
23 ... 34 0.1 140 ... 164 1.0 730 ... 1119 1.9
35 ... 44 0.2 165 ... 190 1.1 1120 ... 1399 2.0
45 ... 55 0.3 191 ... 228 1.2 1400 ... 1699 2.1
56 ... 66 0.4 229 ... 273 1.3 1700 ... 1999 2.2
67 ... 78 0.5 274 ... 320 1.4 2000 ... 2399 2.3
79 ... 90 0.6 321 ... 379 1.5 2400 ... 3199 2.4
91 ... 104 0.7 380 ... 453 1.6 3200 2.5
105 ... 120 0.8 454 ... 561 1.7

### 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:
`Cp` 0.0-0.1 0.2-0.3 0.4-0.5 0.6-0.7 0.8-0.9
`C9` 0 1 2 3 4
`Cp` 1.0-1.1 1.2-1.4 1.5-1.8 1.9 2.0-2.5
`C9` 5 6 7 8 9

For questions concerning specific data or the contents of this page,
please contact:
Kevin Ivory,
mailto:Kevin.Ivory@linmpi.mpg.de

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Last modified: 8 Jul 1997