File: ULYMAG_JUP_1M_FMT.txt Version: NSSDC 1/4/99 JW/JFC
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The following description applies to the Ulysses Magnetometer data files
for the 1992 Jupiter Encounter, submitted in June 1996 by Joyce Wolf for
the Ulysses Magnetometer Team at JPL and Imperial College (A. Balogh,
Principal Investigator).
Files and Data Coverage
The data set covers the period Jan 25 through Feb 18, 1992 (days 25
to 48 inclusive).
Files VHM92xxx.asc and FGM92xxx.asc contain one minute averages of
the magnetic field components and magnitude measured by either the
VHM (Vector Helium Magnetometer) or FGM (Fluxgate Magnetometer),
where xxx = Day of Year (025 = Jan 25). The three days of closest
approach (38-40) are FGM; the others are VHM.
Files TRJ92xxx.asc contain positions and attitude of Ulysses. Time
resolution is the same as the final SEDR: 1 minute for Days 32-47,
3 hours for Days 25-31, 1 hour for Day 48.
File Formats
Each line in the VHM/FGM files contains Year (2 digits), Day of Year,
Hour, Minute, Second, BR, BTHETA, BPHI, BMAGNITUDE in the format
(1x,i2,i4.3, 3i3.2, 4f10.2). The time tag is the midpoint of the
one minute averaging interval. BMAGNITUDE is the average of the
field magnitude, not the magnitude of the average field vector.
Field units are nT. BR, BTHETA and BPHI are one-minute averages
of the field components in R-THETA-PHI coordinates (see below).
Each line of the TRJ files contains Year, Day of year, Hour, Minute,
Second, R, RLATJG, RLONJG, RLATEC, RLONEC, AXISLAT, AXISLON, XSU,
YSU,ZSU in the format (1x,i2,i4.3,3i3.2, f12.5, 2f8.3, 2x, 2f8.3,
2x, 2f8.3,3f12.1). R is the distance from Jupiter to Ulysses
measured in RJ (1 RJ = 71398 km). The next 6 parameters are angles
measured in deg. RLATJG and RLONJG are the latitude and west
longitude of Ulysses in the Jovigraphic System III (1965.0).
RLATEC and RLONEC are the latitude and longitude of the
Jupiter-to-Ulysses unit vector in ECL50 (Earth Mean Ecliptic and
Equinox of 1950.0). AXISLAT and AXISLON are the latitude and
longitude of the Ulysses spin axis in ECL50. XSU, YSU, and ZSU are
the Cartesian components of the Sun-to-Ulysses vector in ECL50.
Coordinate System
The field components are given in the R-THETA-PHI system, which
is that conventionally used for comparison with models. The R axis
is from Jupiter to Ulysses; the THETA axis is perpendicular to R
and lies in the plane containing R and Jupiter's rotation axis and
is positive southward; PHI completes the orthogonal right-handed
system.
The TRJ files contain all the parameters necessary to transform
the field components into System III, ECL50, or inertial spacecraft
coordinates. See Computation of Coordinate Transformations, below.
Data Processing
VHM files were produced by first averaging high resolution (1s or
2s) field data in inertial spacecraft coordinates. Then the averages
were transformed into R-THETA-PHI coordinates, using parameters from
the Final SEDR (Supplementary Experiment Data Records). FGM files
were produced in a similar manner by R.J. Forsyth at Imperial
College; these files were then reformatted at JPL to make the time
tags consistent with those in the VHM files.
TRJ files are simply convenient extractions of parameters from the
Final SEDR.
Accuracy
Since the VHM sensor has greater resolution (4 pT in the +/- 8 nT
range), it was decided that the best quality dataset would consist
of VHM data for the majority of the encounter period and FGM for
days 38-40 when the VHM was in saturation.
We feel that both VHM and FGM datasets have been individually
calibrated to the best level that we can achieve at this time. It
is possible that further small improvements could be made in the
cross calibration between the two sensors; in the present dataset
the differences between VHM and FGM at the beginning of day 38 and
at the end of day 40 are less than 0.5 nT.
For the position of Ulysses in Jupiter-centered coordinates, 1-sigma
uncertainties are less than 1 part in 1 million, or less than 0.5 km
at closest approach.
The attitude of Ulysses during the Jupiter encounter interval is
stated to be accurate within +/- 0.005 degrees.
Computation of Coordinate Transformations
The paragraphs below give methods for computing transformation
matrices using trajectory parameters from the TRJ files. As an
alternative, note that the appendix in [Smith 1993] contains the
orbital elements of Ulysses with respect to Jupiter and demonstrates
how to calculate the position of Ulysses in System III and other
coordinate systems without recourse to trajectory data files.
The transformation matrix from R-THETA-PHI to System III (1965.0)
consists of the column vectors of the R, THETA, and PHI axes
expressed in System III. The R-axis in System III is cos(RLATJG)
cos(360-RLONJG), cos(RLATJG) sin(360-RLONJG), sin(RLATJG). The PHI
axis is the normalized crossproduct J x R, where J is the rotation
axis which is just 0,0,1, so the unit vector in the PHI direction is
-sin(360-RLONJG), cos(360-RLONJG), 0. The unit vector in the THETA
direction is the crossproduct PHI x R = sin(RLATJG) cos(360-RLONJG),
sin(RLATJG) sin(360-RLONJG), -cos(RLATJG).
The transformation matrix from R-THETA-PHI back to ECL50 consists of
the column vectors of the R, THETA, and PHI axes expressed in ECL50.
R is cos(RLATEC) cos(RLONEC), cos(RLATEC) sin(RLONEC), sin(RLATEC).
PHI is the normalized crossproduct J x R, where J (North Pole of
Jupiter) is given in the reference [Smith 1993] as -92.002 RA,
64.504 DEC, Earth Mean Equinox and Equator 1950.0. Rotating by
23.4458 deg (1950.0 obliquity) gives J in ECL50 = (-.015037545,
-.035534090, 0.999255323). The THETA axis is PHI x R.
Inertial spacecraft coordinates are defined as follows: Z is the
Ulysses spin axis, which points approximately towards Earth; X is
is perpendicular to Z and lies in the plane containing Z and S,
where S is the Ulysses-to-Sun vector. X is positive toward the Sun.
Z in ECL50 is cos(AXISLAT) cos(AXISLON), cos(AXISLAT) sin(AXISLON),
sin(AXISLAT). S in ECL50 is -XSU, -YSU, -ZSU. The Y axis is the
normalized crossproduct Z x S, and the X axis is Y x Z. The
transformation matrix from ECL50 back to inertial spacecraft
coordinates consists of the column vectors X, Y, and Z.
The TRJ files in this submission include all the parameters
necessary to calculate the above transformations. In a few cases
where the direction of the spin axis was not available in the SEDR,
the Ulysses-to-Earth direction was substituted in the TRJ files.
It is suggested that interpolations in time be performed on vector
components rather than angles in order to avoid difficulties near
0 or 360, and that double precision arithmetic be used in matrix
multiplication.
References
Smith, E.J., and Wenzel, K.-P. Introduction to the Ulysses
Encounter with Jupiter, J. Geophys. Res., 98, 21111, 1993.
Balogh, A., et al. The magnetic field investigation on the Ulysses
mission: Instrumentation and preliminary scientific results,
Astron. Astrophys. Suppl. Ser., 92, 221, 1992a.
Balogh, A., et al. Magnetic field observations during the Ulysses
flyby of Jupiter, Science, 257, 1515-1518, 1992.
Technical Contact
Joyce Wolf
JPL 169-506
4800 Oak Grove Drive
Pasadena, CA 91109
Phone 818-354-7361
Email jwolf@jplsp.jpl.nasa.gov