# 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.