Geomagnetic disturbances can be monitored by ground-based magnetic observatories recording the three magnetic field components. The global Kp index is obtained as the mean value of the disturbance levels in the two horizontal field components, observed at 13 selected, subauroral stations . The name Kp originates from "planetarische Kennziffer" ( = planetary index).

The following definition of K variations has been given by Siebert (1971):

K variations are all irregular disturbances of the geomagnetic field caused by solar particle radiation within the 3-h interval concerned. All other regular and irregular disturbances are non K variations. Geomagnetic activity is the occurrence of K variations.

Local disturbance levels are determined by measuring the range (difference between the highest and lowest values) during three-hourly time intervals for the most disturbed horizontal magnetic field component. First, however, the quiet-day variation pattern has to be removed from the magnetogram, a somewhat subjective procedure. The range is then converted into a local K index (first introduced 1938 for the magnetic observatory Niemegk near Potsdam) taking the values 0 to 9 according to a quasi-logarithmic scale, which is station specific; this is done in an attempt to normalize the frequency of occurrence of the different sizes of disturbances. But K still remains a local index, describing disturbances in the vicinity of each observatory. According to the geographic and geomagnetic coordinates of the observatories, each observatory still has an annual cycle of daily variations. Using statistical methods, Julius Bartels generated conversion tables to eliminate these effects. By applying the conversion tables, a standardized index Ks for each of the 13 selected observatories is determined. In contrast to the K values, the Ks index is expressed in a scale of thirds (28 values):

0o, 0+, 1-, 1o, 1+, 2-, 2o, 2+, ... , 8o, 8+, 9-, 9o

The main purpose of the standardized index Ks is to provide a basis for the global geomagnetic index Kp which is the average of a number of "Kp stations", originally 11. The Ks data for the two stations Brorfelde and Lovö, as well as for Eyrewell and Canberra, are combined so that their average enters into the final calculation, the divisor thus remaining 11.

Kp was introduced as a magnetic index by Bartels in 1949. Both K and Kp were officially adopted by the International Association for Terrestrial Magnetism and Electricity (IATME, which later became the International Association for Geomagnetism and Aeronomy,IAGA) in 1951 and the series of Kp was extended backwards to 1932 during the subsequent period.

Bartels introduced a musical diagram for a demonstrative presentation of the planetary indices. The musical diagram shows the quantitative values of the Kp. It further enables a quick orientation on the activity by means of the length and the width of the symbols. The repetition of disturbed periods is clearly expressed through the convenient ordering by solar rotations. The musical diagram key is as follows:

During 1949-1996 Kp has been derived at the Institut für Geophysik of Göttingen University, Germany. Since 1997 the Kp and related indices are derived at the Adolf Schmidt Geomagnetic Observatory Niemegk of the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences. Kp and its related indices (ap, Ap, Cp) are widely used in ionospheric and magnetospheric studies and are generally recognized as indices measuring worldwide geomagnetic activity.

The IAGA-97 Resolutions #5 and #6 recognize the value of preserving the uniqueness of the official IAGA indices and the importance of geomagnetic activity indices for characterization and prediction of a wide range of geomagnetic phenomena.

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:

The selection of the quietest days (Q-days) and most disturbed days (D-days) of each month is deduced from the Kp indices on the basis of three criteria for each day:

1. The sum of the eight Kp values.
2. The sum of squares of the eight Kp values.
3. The maximum of the eight Kp values.

According to each of these criteria, a relative order number is assigned to each day of the month, the three order numbers are averaged and the days with the lowest and the highest mean order numbers are selected as the five (respectively ten) quietest and the five most disturbed days.

It should be noted that these selection criteria give only a relative indication of the character of the selected days with respect to the other days of the same month. As the general disturbance level may be quite different for different years and also for different months of the same year, the selected quietest days of a month may sometimes be rather disturbed or vice versa.

In order to indicate such a situation, selected days which do not satisfy certain absolute criteria are marked as follows:

• A selected quiet day is considered not really quiet and is marked by the letter A if Ap greater than 6, or marked by the letter K if Ap less or equal 6 and either one Kp value greater than 3 or two Kp values greater than 2+.
• A selected disturbed day is considered not really disturbed and marked by an asterisk (*) if Ap less than 20.

The sites listed here are those used in the creation of the Kp index, and hence also in the derived ap index. The observatories are listed in the order of their geomagnetic latitude (beginning with the highest one).

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:

JFND

The months around winter solstice: January, February, November and December.

MASO

The months around the equinoxes: March, April, September and October.

MJJA

The months around summer solstice: May, June, July and August.

 

Please find the tables below in PNG format images:

• Meanook (MEA)
• Sitka (SIT)
• Lerwick (LER)
• Eskdalemuir (ESK)
• Uppsala (UPS)
• Lovö (LOV)
• Brorfelde (BFE)
• Rude Skov (RSV)
• Wingst (WNG)
• Niemegk (NGK)
• Witteveen (WIT)
• Hartland (HAD)
• Abinger (ABN)
• Ottawa (OTT)
• Agincourt (AGN)
• Fredericksburg (FRD)
• Cheltenham (CLH)
• Eyrewell (EYR)
• Amberley (AML)
• Canberra (CNB)
• Toolangi (TOO)

• J. Bartels, The technique of scaling indices K and Q of geomagnetic activity, Ann. Intern. Geophys. Year 4, 215-226, 1957;
• J. Bartels,The geomagnetic measures for the time-variations of solar corpuscular radiation, described for use in correlation studies in other geophysical fields, Ann. Intern. Geophys. Year 4, 227-236, 1957;
• M. Siebert, Maßzahlen der erdmagnetischen Aktivität, in Handbuch der Physik, vol. 49/3, 206-275, Springer, Berlin Heidelberg, 1971; for the description of several indices (in German);
• M. Siebert and J. Meyer, Geomagnetic Activity Indices, in The Upper Atmosphere (Eds. W. Dieminger et al.), 887-911, Springer, Berlin Heidelberg, 1996; for an updated version of the German reference;
• P.N. Mayaud, Derivation, Meaning and Use of Geomagnetic Indices, Geophysical Monograph 22, Am. Geophys. Union, Washington D.C., 1980; for a complete review of the present IAGA indices;
• G.K. Rangarajan, Indices of magnetic activity, in Geomagnetism, edited by I.A. Jacobs, Academic, San Diego, 1989;
• M. Menvielle and A. Berthelier, The K-derived planetary indices: description and availability, Rev. Geophys. 29, 3, 415-432, 1991.