| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Voyager magnetometer investigation (P.I.- Norman F. Ness) makes
available archival data through the National Space Science Data Center
(NSSDC) located at NASA/GSFC as well as through the Planetary Data System
(PDS) and other channels. The primary archive format, referred to as a
'summary tape' or 'conjoint summary tape' has been used consistently
since the beginning of the Voyager mission to the outer planets (1977).
This format makes available magnetometer observations, supplementary
engineering and ephemeris data in one data file, and it is one product
of Voyager magnetometer routine data processing. Users are referred to
the summary format data for all data requirements with one exception:
Neptune encounter high field observations. Neptune close approach
observations are archived separately and in a different format from
that with which many are familiar. This high-field archive is
described here.
The special demands of the Neptune encounter flyby exceeded (finally)
the capabilities of the routine data processing system conceived and
implemented in the mid 1970's. As a result, it was necessary to implement
an additional data processing system with which the near-encounter, high
field magnetometer observations were processed. The data products avail-
able with this new data processing system are not available in the same
format as the standard Voyager magnetometer observations; thus the need
for a separate archive and a new format description. We appreciate the
desirability of a consistent archive format, but find no reasonable
alternative to the present solution. We expect, however, that users
interested in near encounter observations will find this new format both
useful and easy to assimilate.
The magnetic fields investigation on Voyager carries a total of four
tri-axial ring core fluxgate magnetometers: two identical high field
magnetometers mounted on the spacecraft body (HFM's) and two identical
low field magnetometers (LFM's) arranged on a 13 m boom (Behannon et
al., 1977). The two LFM's automatically step through a total of eight
dynamic ranges (ranges 0 through 7) in response to changes in the
measured field, starting with a nominal dynamic range of 8 nT, and
increasing to a nominal dynamic range of 50,000 nT. The two HFM's each
operate in two dynamic ranges (ranges 0 and 1) with nominal values of
50,000 nT and 200,000 nT. In the Neptune encounter mode, each
magnetometer was sampled periodically with a temporal resolution of
between 0.600 s and 0.060 s, and analog-to-digital converted with 12 bit
resolution for subsequent telemetry. The magnitude of the maximum
observed field at Neptune (approximately 10,000 nT) was sufficiently
large that data from all four of the magnetometers proved useful.
Parameters
==========
Derived Parameters
------------------
Sampling Parameter Name : time
Sampling Parameter Resolution : 12.0 seconds
Minimum Sampling Parameter : 19770820120000.000000
Maximum Sampling Parameter : UNK
Sampling Parameter Interval : 12.0 seconds
Minimum Available Sampling Interval : 0.060000 seconds
Data Set Parameter Name : magnetic field vector
Noise Level : 0.006000 nT
Data Set Parameter Unit : nanotesla
Magentic field vector: A derived parameter which combines the 3
orthogonal magnetic field component measurements.
Measured Parameters
-------------------
Magentic field component: A measured parameter equaling the
magnetic field strength (e.g. in nanoteslas) along a particular
axis direction. Usually the three orthogonal axis components are
measured by three different sensors.
Data
====
1) MAGNETOMETER DATA PRODUCT
ctime - Decimal seconds past 1966-01-02T00:00:00 (an internal time format)
SCET - Spacecraft event time PDS/ISO standard time format.
B_R - Magnetic field vector radial component in units of nanotesla;
planetocentric right-handed spherical coordinate system.
B_THETA - Magnetic field vector theta component, units nanotesla.
B_PHI - Magnetic field vector phi component, units nanotesla.
The vector magnetic field is rendered in a right handed spherical
coordinate system in which the angles THETA and PHI are the usual
polar angles, with THETA (colatitude) measured from the axis of
rotation and PHI increasing in the direction of rotation. The
orientation of Neptune's pole is specified by a right ascension
of 298.90 and declination of 42.84 at the time of the encounter, as
given in the Jet Propulsion Laboratory's distribution of physical
constants dated 11/06/89. Planetary longitudes are based on a
16.11 hour rotation period (Warwick et al., 1989) adopted by the
Voyager Project shortly after the encounter. The zero longitude is
defined by the requirement that the West Longitude of the spacecraft
at 0356 SCET day 237 (near closest approach) be 167.7 NLS; West
Longitudes of the Neptune Longitude System (NLS) are simply related
to the angle PHI:
WLONG = 360. - PHI (degrees)
This definition of the zero longitude was adopted by the Voyager
Project Steering Group in order to minimize differences in longitudes
resulting from changes in the assumed rotation period.
The field values are instantaneous, not averaged, samples of
'full word' magnetic field observations. These 'full word'
observations are obtained from each of the Voyager low-field (inboard
and outboard) magnetometers and high-field (inboard and outboard)
magnetometers every 0.6 seconds. (Note that 'delta' word samples are
available at much higher sample rates of 16.666 samples/second).
These 'full word' samples are resampled at a time resolution of 12
seconds for present purposes (in the high-field region of the
magnetosphere, these data are highly redundant at 0.6 seconds
resolution). All four triaxial fluxgate magnetometers operated
throughout the encounter. The comprehensive data set is constructed
of inboard low-field magnetometer samples and outboard high-field
magnetometer samples as follows:
SCET 89 236 18 0 0 557
to 89 237 3 44 0 512: inboard low-field magnetometer
SCET 89 237 3 44 12 512
to 89 237 4 2 24 511: outboard high-field magnetometer
SCET 89 237 4 2 36 511
to 89 238 8 19 48 379: inboard low-field magnetometer
This procedure was adopted as a result of an increased level of
spacecraft interference and telemetry anomalies experienced during the
Neptune encounter by MAG. The composite set of observations has good
time continuity. Spacecraft zeros are assumed constant throughout the
interval represented by this data set, and for that reason users of
low-field data are referred to the summary tape format (standard
archive format) in which greater care is exercised with the estimation
of time-variable spacecraft zeros (differences of 0.05 nT or less). This
data set extends from inbound MP to outbound MP for the convenience of
users who do not require better weak-field accuracy than that quoted
above.
2.) INTERNAL - A BINARY file of observations and supplementary ephemeris
data organized specifically for analysis of the planetary magnetic field
(Connerney, Acuna, and Ness, Journal of Geophysical Research, 1991).
These data are resampled 'full word' observations of inboard low-field
magnetometer and outboard high-field magnetometer observations according
to the schedule given above. These data are a subset of the Voyager 2
vector magnetic field observations obtained within 12 Rn radial distance
of Neptune (SCET day 236 hr 2340 to SCET day 237 0811). During this time
the field magnitude ranged from approximately 10 nT to nearly 10,000 nT.
These data were resampled (decimated in time without averaging) to produce
samples every 48, 24, and 12 seconds, with the highest temporal resolution
reserved for the closest approach interval (where Voyager 2's motion
relative to the planet was greatest). Note that the format of this ASCII
file is different from that described above; a segment of the file is
reproduced here, again beneath column headings inserted here only:
RADIUS THETA PHI B_COMPONENT SIGMA TYPE
----------------------------------- ---------------------------------
1.349 0.685 4.614 6902.570 3.500 0
1.349 0.685 4.614 3598.230 3.500 1
1.349 0.685 4.614 -2457.560 3.500 2
1.339 0.669 4.602 7153.850 3.500 0
1.339 0.669 4.602 3555.870 3.500 1
1.339 0.669 4.602 -2591.020 3.500 2
1.334 0.661 4.595 7275.760 26.00 0
1.334 0.661 4.595 3540.800 26.00 1
1.334 0.661 4.595 -2623.690 26.00 2
1.328 0.652 4.589 7401.400 26.00 0
1.328 0.652 4.589 3506.690 26.00 1
1.328 0.652 4.589 -2720.230 26.00 2
RADIUS - Spacecraft radial distance in units of Rn
( 1 Rn = 24,765 km)
THETA - Theta coordinate of spacecraft in units of radians
PHI - Phi coordinate of spacecraft in units of radians
B_COMPONENT - Magnetic vector field component in units of nanotesla
according to the value of identifier TYPE
TYPE - Magnetic field value designation
TYPE = 0: Radial component
TYPE = 1: Theta component
TYPE = 2: Phi component
TYPE = 3: Magnitude of magnetic field
SIGMA - Estimated standard deviation of the observation in
units of nanotesla.
Each vector observation was weighted with the expected standard
deviation of the observation in the analysis of the internal field.
This practice insures that each residual (the difference between an
observation and a model field) is compared with its expected error.
For the standard deviation of each measurement, we adopted 1 nT or
the nominal quantization stepsize of the measurement, whichever was
greater. The former value approximates the variable magnetospheric
magnetic field 'noise', the unmodeled temporal and spatial
variations in the ambient field. The latter approximates the
estimated measurement error ( 0.05 nT + 0.1% of full scale). The
quantization step size of the measurement is a function of the
ambient field strength (or time), since the Voyager magnetometers
automatically change range in response to changes in the ambient
field. In ranges 0 through 5, the quantization stepsize of the
lfm measurement increased from a nominal value of 0.005 nT to
1.1 nT. In ranges 6 and 7 (the highest two ranges) each lfm was
sampled with a quantization stepsize of approximately 3.5 and 26
nT, respectively. Each hfm, operating in range 0 throughout the
encounter, was sampled with a quantization of approximately 26 nT.
Therefore, weights of 1 nT were used for observations obtained in
the lower instrument ranges, from SCET day 236/2340 to day 237/0330
and from day 237/0431 to 237/0811. Weights of 3.5 nT were used for
observations obtained in lfm range 6, from SCET day 237/0330 to
237/0344 and from day 237/0402 to 237/0431. Weights of 26 nT were
used for the hfm observations, from SCET day 237/0344 to 237/0402.
The segment of data illustrated above includes the transition to hfm
observations with an associated estimated standard deviation of 26
nanoteslas. The sampling interval was also adjusted according to
instrument range, with a 48 second sampling interval for lfm
range 5 observations, a 24 second sampling interval for lfm range 6
observations, and a 12 second sampling interval for hfm
observations.
3.) INTERNAL FIELD MODEL COEFFICIENTS - electronic form at
A Neptune spherical harmonic magnetic field model (Connerney et
al., 1991) is listed here in electronic format for use in magnetic
field calculations. The model is based upon a partial solution (44ev)
to an 8th order expansion of the internal field, as is necessary to
adequately represent the field measured by Voyager close to the
surface of Neptune. However, few of the model coefficients are
resolved, or uniquely determined; these few are marked with an
asterisk (*). Some additional coefficients are marginally resolved;
these are marked with a # sign. The remaining coefficients are not
resolved, which means that they are not constrained by the data,
and may assume practically any value. They are, however, necessary
to represent the field along the Voyager trajectory close to
the planet. USERS ARE ENCOURAGED TO READ CAREFULLY THE DISCUSSION
IN THE NEPTUNE SPECIAL ISSUE OF JGR REGARDING MODEL PARAMETER
RESOLUTION AND USE OF THE MODEL COEFFICIENTS.
Beyond about 2.5 or 3 Rn, where higher order terms are sufficiently
attenuated, the quadrupole approximation is a good approximation to
the field globally. These terms are (more or less) uniquely
determined. At close-in radial distances, our description of the
field is necessarily incomplete. The entire expansion to eighth
degree and order provided will well describe the field in the
vicinity of the Voyager trajectory. Those parts of the field that
the data are insensitive to are effectively zero along the Voyager
trajectory. As one strays further from the Voyager trajectory in
close to the planet, the real field may be expected to (increasingly)
deviate from that calculated with the entire expansion. Given
Voyager's close approach (1.18 Rn) to the planet, any low-order
approximation to the field is expected to be inaccurate, even on
the spacecraft trajectory (Figure 9 in the paper by Connerney
et al. illustrates this problem). For use in modeling the field
near the planet, (at radial distances of less than approximately
2 or 3 Rn), an octupole approximation to the field is required to
even grossly approximate the field. Unfortunately, many of these
coefficients are not well resolved.
For global field modeling near the planet, we advocate use of a
model field consisting of orders 1, 2 and 3 (dipole, quadrupole,
and octupole) from Table 1. This model, a subset of the coefficients
listed below, is referred to as the 'O8' model (the octupole (O)
part of an eighth degree and order spherical harmonic expansion).
This decision reflects a compromise between the limited model
parameter resolution afforded by the Voyager trajectory and the
anticipated demands on the model. However, some of the quadrupole
and octupole parameters are not well resolved and we expect these
to be inaccurate. This is an inescapable consequence of the limited
sinformation content of the observations.
-------------------------------------------------------------------
Table 1. Neptune I8E1 44ev Magnetic Field Model
Schmidt-Normalized Spherical Harmonic Coefficients
Neptune Radius = 24,765 km
-------------------------------------------------------------------
n m g(n,m) h(n,m)
-------------------------------------------------------------------
1 0 0.09732 *
1 1 0.03220 * -0.09889 *
2 0 0.07448 #
2 1 0.00664 # 0.11230 *
2 2 0.04499 * -0.00070 *
3 0 -0.06592 *
3 1 0.04098 -0.03669 #
3 2 -0.03581 0.01791 #
3 3 0.00484 # -0.00770 #
4 0 0.02243
4 1 0.00557 -0.01889 #
4 2 0.03099 0.02607
4 3 -0.01287 0.01204
4 4 -0.05073 -0.00456
5 0 -0.00202
5 1 -0.00229 -0.00739
5 2 0.00526 -0.01134
5 3 -0.02846 0.01067
5 4 -0.01425 -0.01551
5 5 -0.02835 -0.01090
6 0 -0.02175
6 1 -0.00466 0.04432
6 2 -0.01269 -0.01598
6 3 -0.02233 0.01721
6 4 -0.00887 0.00370
6 5 -0.00496 -0.01932
6 6 0.00755 0.01439
7 0 0.01671
7 1 0.01678 -0.03159
7 2 0.01625 0.01862
7 3 0.02157 -0.01120
7 4 -0.00483 0.00515
7 5 0.01873 0.01923
7 6 0.00584 -0.02749
7 7 0.00664 0.03344
8 0 -0.00689
8 1 0.00238 0.01446
8 2 -0.00090 -0.00079
8 3 -0.01304 0.01043
8 4 0.00311 -0.00022
8 5 -0.00367 -0.00465
8 6 -0.00249 0.01043
8 7 0.01333 -0.02138
8 8 -0.01239 0.02519
* = Coefficient WELL RESOLVED (Rxx > 0.95)
# = Coefficient MARGINALLY RESOLVED (0.75 < Rxx < 0.95)
ALL OTHER COEFFICIENTS are POORLY RESOLVED or UNRESOLVED
Refer to text of Connerney et al., 1991, for explanation.
Coordinate System
=================
Neptune West Longitude System (NLS) Coordinates
-----------------------------------------------
COORDINATE SYSTEM CENTER : NEPTUNE
COORDINATE SYSTEM REFERENCE EPOCH : UNK (1989-08-25:03:56:00.000)
The orientation of Neptune's pole is specified by a
right ascension of 298.90 and declination of 42.84 at the time
of the encounter, as given in the Jet Propulsion Laboratory's
distribution of physical constants
dated 11/06/89. Planetary longitudes are based on a
16.11 hour rotation period (Warwick et al., 1989) adopted
by the Voyager Project shortly after the encounter.
The zero longitude is defined by the requirement that the West
Longitude of the spacecraft at 0356 SCET
day 237 (near closest approach) be 167.7 NLS;
West Longitudes of the Neptune Longitude System (NLS)
are simply related to the angle PHI:
WLONG = 360. - PHI (degrees)
This definition of the zero longitude was adopted by the
Voyager Project Steering Group in order to minimize
differences in longitudes resulting from changes in the
assumed rotation period.
Magnetic-field is defined in terms of the following:
R - Radial along the Neptune-S/C line, positive away from
Neptune
Phi - east longitudinal component
Theta - colatitudinal component
Position is given in terms of the following:
RANGE (R) - Range from the planet center to the
spacecraft in units of Rn where
Rn = 24,765km.
LATITUDE (LAT) - Spacecraft latitude in degrees. Valid
range -90.0 -> +90.0.
LONGITUDE (W_LONG) - West longitude where the zero longitude is
defined by the requirement that the West
Longitude of Voyager 2 at
1989-08-25T03:56:00.00 was 167.7 degrees.
References
==========
Behannon, K. W., M. H. Acuna, L. F. Burlaga, R. P. Lepping, N. F. Ness,
and F. M. Neubauer, 'Magnetic field experiment for Voyagers 1 and 2',
Space Sci. Rev., 21, 235-257, 1977.
Connerney, J. E. P., M. H. Acuna, and N. F. Ness, 'The magnetic field
of Neptune', J. Geophys. Res., 96, Supplement, 19023-19042, 1991.
Ness, N. F., M. H. Acuna, L. F. Burlaga, J. E. P. Connerney, R. P.
Lepping, and F. M. Neubauer, 'Magnetic fields at Neptune', Science, 246,
1473-1478, 1989.
Warwick, J. W. et al., 'Voyager planetary radio astronomy at Neptune',
Science, 246, 1498-1501, 1989.
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