
SSC Users Guide  Table of Contents
1. GEI: Geocentric Equatorial Inertial system. This system has Xaxis pointing from the Earth toward the first point of Aries (the position of the Sun at the vernal equinox). This direction is the intersection of the Earth's equatorial plane and the ecliptic plane and thus the Xaxis lies in both planes. The Zaxis is parallel to the rotation axis of the Earth, and y completes the righthanded orthogonal set (Y = Z * X). Geocentric Inertial (GCI) and EarthCentered Inertial (ECI) are the same as GEI.
2. GEO: Geographic coordinate system. This system is defined so that its Xaxis is in the Earth's equatorial plane but is fixed with the rotation of the Earth so that it passes through the Greenwich meridian (0 longitude). Its Zaxis is parallel to the rotation axis of the Earth, and its Yaxis completes a right handed orthogonal set (Y = Z * X).
3. GM: Geomagnetic coordinate system. Zaxis points to the Geomagnetic north pole (in Greenland). The positive Xaxis points towards the great circle encompassing the North and South Geomagetic poles and lies in the geomagnetic equatorial plane in the segment that is in the western hemisphere. (The South GM pole is the antipode of the North GM pole.) Earthcentered Dipole is invoked. Y completes the triad.
4. GSE: Geocentric Solar Ecliptic system. This has its Xaxis pointing from the Earth toward the Sun and its Yaxis is chosen to be in the ecliptic plane pointing towards dusk (thus opposing planetary motion). Its Zaxis is parallel to the ecliptic pole. Relative to an inertial system this system has a yearly rotation.
5. GSM: Geocentric Solar Magnetospheric system. This has its Xaxis from the Earth to the Sun. The Yaxis is defined to be perpendicular to the Earth's magnetic dipole so that the XZ plane contains the dipole axis. The positive Zaxis is chosen to be in the same sense as the northern magnetic pole. The difference between the GSM and GSE systems is simply a rotation about the Xaxis.
6. SM: Solar Magnetic coordinates. In this system, the Zaxis is chosen parallel to the north magnetic pole and the Yaxis perpendicular to the EarthSun line towards dusk. The difference between this system and the GSM system is a rotation about the Yaxis. The amount of rotation is simply the dipole tilt angle. We note that in this system the Xaxis does not point directly at the Sun. As with the GSM system, the SM system rotates with both a yearly and daily period with respect to inertial coordinates.
7. Invariant Latitude: For any point in space one can trace a Bfield line to the Earth surface, assuming it is a centered dipole field. The GM latitude of this foot point is labelled as the Invariant Latitude along the entire field line. The dipole Lvalue is closely related to this invariant latitude; L=1/(Cos(Lat))^2, and physically connotes the distance (in Earth radii) of the "top of the field line" from Earth center.
8. J2000: Geocentric Equatorial Inertial for epoch J2000.0 (GEI2000), also known as Mean Equator and Mean Equinox of J2000.0 (Julian date 2451545.0 TT (Terrestrial Time), or 2000 January 1 noon TT, or 2000 January 1 11:59:27.816 TAI or 2000 January 1 11:58:55.816 UTC.) This system has Xaxis aligned with the mean equinox for epoch J2000; Zaxis is parallel to the rotation axis of the Earth, and Y completes the righthanded orthogonal set.
Reference:"Geophysical Coordinate Transformations", C.T. Russell, Cosmic Electrodynamics, Vol. 2, pp. 184  196, 1971
SSC Users Guide  Table of Contents