Note: Heavy (dark) line is for the current 27-day Bartel rotation; light line is for the last rotation.
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 an estimate of Ap generated by the USAF Weather Agency Space Weather Operation Center (AFWA/XOGX, located at Offutt AFB, NE) from a different, smaller set of stations than used in the final Ap values. The AFWA/XOGX Ap values are available in several products issued by the NOAA SWPC.
References for the Ap index are as follows:
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