DESCRIPTION OF VOYAGER MAG DATA SUBMISSIONS TO THE NSSDC

The high resolution data submitted to the NSSDC has its origins in the 
original MVS "Summary" data sets formally produced on an IBM MVS 
mainframe system.  The original data sets contain a mix of engineering, 
electron, magnetic field and plasma data.  The Voyager magnetometer (MAG)
experiment now resides on a dedicated workstation where only MAG data is
processed.  High resolution MAG data continues to use the same format as 
that of archived MVS data sets.

All Voyager MAG data submitted to the NSSDC consists wholly of high resolution 
LFM averages.  These files consist of a set of averages applied across all 
of the mission's differing telemetry modes.  In the case of the Magnetometer 
Experiment, the records contain both 1.92 second, 9.6 second and 48 second
averages.  1.92 second averages are created from the detail data,
9.6 second averages are created from 1.92 second averages and 48 second 
averages are created from the 9.6 second averages.  Thus, there are 25 1.92
second averages, 5 9.5 averages and one 48 second average per record.

In producing this submission to the NSSDC, the magnetic field records have 
been stripped from MVS "Summary" files and converted from MVS to VAX
binary representation.  File content remains unchanged.  A second, ASCII 
format data set containing key components of the 48 second magnetic field 
data and ephemeris data has been created allowing more convenient access 
to high resolution Voyager magnetometer data.

COORDINATE SYSTEMS

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.

VOYAGER 48 SECOND AVERAGED ASCII DATA FORMAT

field  descriptor
-----  ----------
  1.   s/c identification (FLT1=Voyager 1)
                          (FLT2=Voyager 2)

  2.   coordinate system  (PL=payload)
                          (HG=heliographic)
                          (S3=Jupiter)
                          (L1=Saturn)
                          (U1=Uranus)
                          (N1=Neptune)

  3.   field magnitude (nT)
  4.   field component 1 (nT)
  5.   field component 2 (nT)
  6.   field component 3 (nT)
  7.   spacecraft radial distance (AU)
  8.   spacecraft x position (AU)
  9.   spacecraft y position (AU)
 10.   spacecraft z position (AU)