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Voyager Interplanetary Exploration

Magnetic Field Science

The Voyager MAG experiment was created to investigate basic concepts and dynamical processes in both the planetary and interplanetary magnetic fields. Much of the Voyagers' exploration of magnetic fields has been in the interplanetary domain. With planetary encounters now past, the Voyagers are providing an unprecedented view of the interplanetary field in the distant heliosphere at distances beyond 40 A.U. The theory that serves to explain and unify the interplanetary magnetic field and plasma observations relating to phenomena with scales greater than 100 km is magneto-hydrodynamics (MHD).

Topics of Interest Include:

Radial, Latitudinal and Solar Cycle Variations in the Magnetic Field

Large-Scale Fluctuations

Sectors and Sector Boundaries

Merged Interaction Regions

Shocks

Large-Scale Fluctuations

Pressure Balanced Structures and Interstelar Pickup Protons

Those subjects and the related Voyager observations are reviewed in the book Burlaga, L. F., Interplanetary Magnetohydrodynamics, Oxford University Press, 1995.

Coordinate Systems

The 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 coordinate 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 is 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.

The magnetic field orientation is defined in relation to the position of 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. The three orthogonal axes defined by this system are X(HG),Y(HG), and Z(HG). The X(HG) axis points radially away from the Sun and 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.

A reference to the coordinate systems utilized by the Voyager Project may be found in Burlaga, L. F., Space and Science Reviews, MHD Processes in the Outer Heliosphere,pages 255-316, 1984.