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: