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 Potsdam Adolf-Schmidt-Observatory for Geomagnetism, D-14823 Niemegk (Germany), and are available from http://www.gfz-potsdam.de/pb2/pb23/GeoMag/niemegk/kp_index/ 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:

• 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.

NWRA Space Weather Links

[ Space Weather Home Page | Scintillation Home Page ]

[ Home | Site Map | Company | Staff | Research Areas | Services | Products ]

How to contact NWRA

Copyright © 1996-2011 NorthWest Research Associates, Inc.  All rights reserved.