The Ap index is a measure of the general level of geomagnetic activity over the globe for a given (UT) day. It is derived from measurements made at a number of stations world-wide of the variation of the geomagnetic field due to currents flowing in the earth's ionosphere and, to a lesser extent, in the earth's magnetosphere. The official values for Ap (and other related indices of geomagnetic activity such as the three-hour Kp index) are calculated by the GeoForschungsZentrum Helmholtz Centre Potsdam (Germany) with near real-time "quicklook" values and definitive values updated twice a month (once during the days 16-20 with the first 15 days of the current month and once during the days 1-5 with the second half of the previous month). The values plotted here are provisional Ap based on the quicklook Kp values from their website.

References for the Ap index are as follows:

• Rostoker, G., Geomagnetic Indices, Rev. Geophys. Space Phys., 10, 157, 1972.
• Mayaud, P.N., Derivation, Meaning, and Use of Geomagnetic Indices, Geophysical Monograph 22, American Geophysical Union, Washington, DC, 1980.