DOI: 10.48322/ctar-4d05
title: Voyager 1 9.6-s Averaged Triaxial Fluxgate Magnetometer (MAG) Interplanetary Magnetic Field in Binary Format

description: This magnetic field data set contains 9.6 second averages that are created from 1.92 second averages. It includes the Voyager spacecraft number (1 or 2), the date-time in
decimal year (90.00000 is day 1 of 1990), the magnetic field strength F1
computed from high-resolution magnitudes, the elevation and azimuth angles
(degrees) in heliographic (RTN) coordinates, and the magnetic field strength F2
computed from hour averages of the components. The vector components of B can be
computed from F2 and the two angles. Elevation angle is the latitude angle above
or below the solar equatorial plane, and azimuth angle is in the direction
orbital motion around the Sun from the projection of the Sun-to-spacecraft axis
into the solar equatorial plane. The Voyager MAG experiment and coordinates are
further described in the following publication:  Behannon, K.W., M.H. Acuna,
L.F. Burlaga, R.P. Lepping, N.F. Ness, and F.M. Neubauer, Magnetic-Field
Experiment for Voyager-1 and Voyager-2, Space Science Reviews, 21 (3), 235-257,
1977...At the time of experiment proposal, it was expected that the required
accuracy of the measurements would be 0.1 nT, determined by the combined noise
of the sensors and the spacecraft field. The spacecraft magnetic field at the
outboard magnetic field sensor, referred to as the primary unit, was expected to
be 0.2 nT and highly variable, consistent with current estimates. Hence, the
dual magnetometer design (Ness et al., 1971, 1973; Behannon et al. 1977). ..At
distances > 40 AU, the heliospheric magnetic fields are generally much weaker
than 0.4 nT; the average magnetic field strength near 40 AU and 85 AU is about
0.15 nT and 0.05 nT, respectively. The use of roll calibrations lasting about 6
hours permits determination of the effective zero levels for the two independent
magnetic axes that are perpendicular to the roll axis (which is nearly parallel
to the radius vector to the Sun) at intervals of about 3 months. There is no
roll calibration for the third magnetic axis. Comparison of the two derived
magnetic vectors from the two magnetometers permits validation of the primary
magnetometer data with an accuracy of 0.02 to 0.05 nT. A discussion of the
uncertainties that must be considered when using these data is given in the
Appendix of Burlaga et al. [1994] and in Appendix A of Burlaga et al. [2002].
..References: ..Behannon, K.W., M.H. Acuna, L.F. Burlaga, R.P. Lepping, N.F.
Ness, and F.M. Neubauer, Magnetic-Field Experiment for Voyager-1 and Voyager-2,
Space Science Reviews, 21 (3), 235-257, 1977. ..Burlaga, L.F., Merged
interaction regions and large-scale magnetic field fluctuations during 1991 -
Voyager-2 observations, J. Geophys. Res., 99 (A10), 19341-19350, 1994.
..Burlaga, L.F., N.F. Ness, Y.-M. Wang, and N.R. Sheeley Jr., Heliospheric
magnetic field strength and polarity from 1 to 81 AU during the ascending phase
of solar cycle 23, J. Geophys. Res., 107 (A11), 1410, 2002. ..Ness, N., K.W.
Behannon, R. Lepping, and K.H. Schatten, J. Geophys. Res., 76, 3564, 1971.
..Ness et al., 1973
At distances > 40 AU, the heliospheric magnetic fields are generally much weaker
than 0.4 nT; the average magnetic field strength near 40 AU and 85 AU is about
0.15 nT and 0.05 nT, respectively. The use of roll calibrations lasting  about 6
hours permits determination of the effective zero levels for the two independent
magnetic axes that are perpendicular to the roll axis (which is nearly parallel
to the radius vector to the Sun) at intervals of about 3 months. There is no
roll calibration for the third magnetic axis. Comparison of the two derived
magnetic vectors from the two magnetometers permits validation of the primary
magnetometer data with an accuracy of 0.02 to 0.05 nT. A discussion of the
uncertainties that must be considered when using these data is given in the
Appendix of Burlaga et al. [1994] and in Appendix A of Burlaga et al. [2002].
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.