Kp Index

A third magnetic index, called the Planetary Magnetic Disturbance Index, Kp, (Figure 4.19) has been in use since the early part of the twentieth century. K values at an observatory are obtained from average 3-hr ranges (highest minus lowest values) of the northward field components after an estimate of the quiet-time variation is removed. The 3-hr range scaling discriminates against gradual field changes that are longer than 3 hours, of the type illustrated by the recovery phase of the Dst index. The small short-period variations contribute little. An average of the K indices at selected global stations is Kp, the planetary form of the index. Unfortunately, such stations are not uniformly

Time (UT hours)

FIGURE 4.18 ► Dst Index values for two days of a magnetic storm period. The index is formed as a latitude-adjusted average of the four low-latitude station records illustrated in Figure 4.9.

Time (UT hours)

FIGURE 4.18 ► Dst Index values for two days of a magnetic storm period. The index is formed as a latitude-adjusted average of the four low-latitude station records illustrated in Figure 4.9.

distributed around the Earth. More are in the Northern Hemisphere, there is a concentration in Europe, and many index stations are located at high latitudes just south of the northern auroral zone.

The K index has values from 0 to 9 (with subscripts of 0, +, and — to indicate intermediate units in thirds) that are assigned from tables that convert gammas to Kp. The K numbers are selected so that approximately a 10-times field (in gammas) increase occurs for each whole K number increase (called a pseudologarithmic change, the letter "K" refers to a German word for a logarithmic index). Each contributing station has a different K table, fixed so that, in a long-term average, the number of occasions that each index value appears is the same for each location. Statistically, the most Kp values occur near a value of 2o. Values of 5_ and above are usually magnetic storms. Major storms produce indices of 8_ and above.

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