In many ways similar to the sunspot number, and well correlated, the solar flux index is a measure of solar radio flux at a frequency of 2800MHz, or 10.7 cm as it is commonly called. This measure was introduced in 1947 at Ottawa, Canada and has obvious advantages over the sunspot number in that it does not rely on visual, often subjective observations.
K indices (K, Kp)
Quasi-logarithmic index of geomagnetic activity relative to quiet levels for a local recording station. These measurements are taken over a 3-hour period and reported on a scale of 0 to 9. Planetary (Kp) values are determined from data from 12 to 13 stations worldwide. This indice was begun in 1949 at the Institut für Geophysic, Göttingen University, Germany.
A indices (a, A, ap, and Ap)
Indices derived from the K index but converted to a linear scale as follows:
K 0 1 2 3 4 5 6 7 8 9 a 0 3 7 15 27 48 80 140 240 400The Ap, or planetary A index is an average daily report which I have commonly used to indicate geomagnetic conditions. The equavelant values are defined thus:
Ap Condition 0-7 Quiet 8-15 Unsettled 16-29 Active 30-49 Minor Geomagnetic Storm 50-100 Major Geomagnetic Storm >100 Severe Geomagnetic Storm