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K Indices of Magnetic Activity Relative to an assumed Quiet-day for each 3-Hour Period
The National Geophysical Data Center receives the K indices on a current basis from approximately 85 observatories. The K index is a quasi-logarithmic local index of the 3-hourly range in magnetic activity relative to an assumed quiet-day curve for the recording site. First introduced by J. Bartels in 1938, it consists of a single digit 0 through 9 for each 3-hourly interval. The K indices are received on a monthly basis from the various observatories and are available from 1938 to present. The National Geophysical Data Center also receives KP indices from the Institute for Geophysik in Gottingen, Germany on a monthly basis. The KP index is a planetary index based on the K index from 12 or 13 selected stations and has been derived routinely from 1932.