Kp Index

Geomagnetic disturbances (storms) can be monitored by ground-based magnetic observatories recording the horizontal magnetic field components. The global or planetary Kp index is obtained as the mean value of the disturbance levels observed at 12 selected, subauroral stations. 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 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 taking the values 0 to 9 according to a pseudo-logarithmic scale, which is station specific; this is done in an attempt to normalize the frequency of occurrence of the different sizes of disturbances. The three-hourly Kp index (average of local K values from 12 stations) is expressed in a scale of thirds (28 values):
00, 0+, 1-, 10, 1+, 2-, 20, 2+, ... ,80, 8+, 9-, 90
Kp was introduced as magnetic index by J. Bartels in 1938 and has been derived since then at the Geophysical Institute of Göttingen University, Federal Republic of Germany. A daily index Kp is obtained by summing the eight three-hourly values for that day. Kp and its related indices (ap, Ap, Cp) have been widely used in ionospheric and atmospheric studies and are generally recognized as indices measuring the effect of energetic charged particles arriving in Earth's upper atmosphere after periods of intense solar activity. Km is an index derived similarly as Kp except the number and selection of stations is different. Km is evaluated at the Institute de Physique du Globe in Paris, France.
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