Voyager 2 1-hr Averaged Triaxial Fluxgate Magnetometer (MAG) Interplanetary Magnetic Field in ASCII Format

Coordinate System ================= Interplanetary magnetic field studies make use of two important coordinate systems, the Inertial Heliographic (IHG) coordinate system and the Heliographic (HG) coordinate system. The IHG coordinate system is use to define the spacecraft's position. The IHG system is defined with its origin at the Sun.

There are three orthogonal axes, X(IHG), Y(IHG), and Z(IHG). The Z(IHG) axis points northward along the Sun's spin axis. The X(IHG) - Y(IHG) plane lays in the solar equatorial plane.

The intersection of the solar equatorial plane with the ecliptic plane defines a line, the longitude of the ascending node, which is taken to be the X(IHG) axis. The X(IHG) axis drifts slowly with time, approximately one degree per 72 years. Magnetic field orientation is defined in relation to the spacecraft.

Drawing a line from the Sun's center (IHG origin) to the spacecraft defines the X axis of the HG coordinate system. The HG coordinate system is defined with its origin centered at the spacecraft. Three orthogonal axes are defined, X(HG), Y(HG), and Z(HG).

The X(HG) axis points radially away from the Sun and the Y(HG) axis is parallel to the solar equatorial plane and therefore parallel to the X(IHG)-Y(IHG) plane too. The Z(HG) axis is chosen to complete the orthonormal triad. An excellent reference guide with diagrams explaining the IHG and HG systems may be found in Space and Science Reviews, Volume 39 (1984), pages 255-316, MHD Processes in the Outer Heliosphere, L. F. Burlaga [BURLAGA1984].