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## The AAGCM Coordinate System

When studying phenomena in the earth's magnetosphere and ionosphere, the coordinates of the data are best translated into a coordinate system based on the magnetic field. Especially when working at high latitudes and when comparing satellite with ground based measurements (i.e. DMSP and SuperDARN) one requires a magnetic coordinate system that is smooth at the poles and accurate at different heights. The Altitude Adjusted Corrected Geomagnetic (AACGM) coordinate system is a synthesis of the Epoch Corrected Geomagnetic Coordinate system (CGM) and the Polar Anglo-American Conjugate Experiment (PACE) geomagnetic coordinate system. It has been developed especially for the purpose of correlating effects caused by charged particles with ground based observations [BW89, BH94].

In the past a geomagnetic coordinate system was used that was based on a centered but tilted dipole representation of the earth's magnetic field. For more precise measurements, a corrected magnetic coordinate system devised by Hultqvist (1958) took into account higher order terms in the spherical harmonic expansion of the magnetic field. A real field line through any point on the earth's surface may be traced to the centered dipole equator. The point is defined to be equivalent to a line trace in the multipole field back to the surface. The latitude and longitude of that point in dipole coordinates are the desired "corrected" geomagnetic coordinates. Gustafsson (1970 and 1984) has provided revised corrected geomagnetic coordinates based on the current revision of the International Geomagnetic Reference Field (IGRF) [Ped82]. Since the actual magnetic field changes with time, it is necessary to generate new tables for each Epoch. This was done most recently for the Epoch 1995. CGM coordinates are only defined at ground level so a method based on interpolation was needed to adjust for the altitude. The resulting model is the AACGM coordinate system as described by Baker and Wing [BW89] as well as Bhavnani and Hein [BH94].

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Andreas Schiffler
Wed Oct 9 10:05:17 CST 1996