| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set includes calibrated magnetic field data acquired by
the Voyager 1 Low Field Magnetometer (LFM) during the Saturn
encounter. Coverage begins in the solar wind inbound to Saturn and
continues past the last outbound bowshock crossing. The data are in
Heliographic (RTN) coordinates and have been averaged from the 9.6
second summary rate to a 48 second sample rate. All magnetic field
measurements are given in nanoTesla (nT). The magnetic field data
are calibrated (see the calibration description included in the
Voyager 1 Magnetometer instrument catalog file for details).
Parameters
==========
The full LFM instrument sample rate is 1 sample per 0.06 seconds.
Full telemetry resolution 'detail' data must be obtained from the
instrument team. For this data set, the data have been resampled to
48 seconds from 9.6 second averages. The 9.6 second data were
resampled from 1.92 second averages which were in turn resampled
from the detail data.
The LFM has eight dynamic ranges. The instrument is designed switch
between dynamic ranges automatically depending upon the observed
magnetic field magnitude and fluctuations. Instrument digitization
uncertainty depends upon dynamic range as indicated in the following
table (from [BEHANNONETAL1977]).
-----------------------------------------------
LFM Dynamic ranges and quantization uncertainty
-----------------------------------------------
Range (nT) Quantization (nT)
-----------------------------------------------
1. +/- 8.8 +/- .0022
2. +/- 26 +/- .0063
3. +/- 79 +/- .019
4. +/- 240 +/- .059
5. +/- 710 +/- .173
6. +/- 2100 +/- .513
7. +/- 6400 +/- 1.56
8. +/- 50,000 +/- 12.2
Processing
==========
Voyager EDR's undergo the following processing in order to produce
these 48 second averaged summary data:
* Read EDR
* Unpack header block (rec. id, s/c id, tel. mode, FDS counts,
data flags)
* Convert selected time tags to integer time (yy/ddd/hh:mm:ss.fff)
* Unpack sub-header block (MAG status words, plasma data)
* Unpack science block (MAG counts)
* Convert counts to gammas
* Apply sensor and boom alignment matrices
* Rotate (optional) 1.92 second averages while averaging detail
gammas to create 1.92 second averages
* Average 1.92 second data to 9.6 seconds, then 9.6 second data to
48 seconds
* Write Summary record
Counts are measured onboard using 12 bit words that may represent
values ranging from 0-4096. Integer counts are converted to magnetic
field units (gammas) by subtracting a zero offset, from the measured
MAG value and multiplying this difference by the sensitivity of the
instrument.
Data
====
The data files are given in ASCII, fixed field width, comma
delimited tables. The record structure is described in the following
table:
--------------------------------------------------------------------
48 Second Heliographic (RTN) Coordinates
--------------------------------------------------------------------
Column Type Description
--------------------------------------------------------------------
time a23 spacecraft event time (UT) of the sample in the
format: yyyy-mm-ddThh:mm:ss.sss
sclk a12 spacecraft clock in the format:
MOD65536:MOD60:FDS-LINE
mag_id i1 magnetometer ID (1 = LFM, 2 = HFM)
Br f9.3 average of detail magnetic field R component in nT
Bt f9.3 average of detail magnetic field T component in nT
Bn f9.3 average of detail magnetic field N component in nT
Bmag f9.3 magnitude of the averaged magnetic field components
in nT
avg_Bmag f9.3 average of the magnetic field magnitude over the
averaging interval in nT
Delta f7.3 spacecraft centered heliographic magnetic field
latitude in degrees: Delta = asin(Bn/Bmag)
Lambda f7.3 spacecraft centered heliographic magnetic field
longitude in degrees: Lambda = atan(Bt/Br)
rms_Br f8.3 root-mean-square deviation of Br over the averaging
interval in nT
rms_Bt f8.3 root-mean-square deviation of Bt over the averaging
interval in nT
rms_Bn f8.3 root-mean-square deviation of Bn over the averaging
interval in nT
npts i2 number of detail points in the average
dflag a8 8-byte character string which indicates software or
spacecraft hardware intervention which reduces
confidence in the data
[Note that the magnetic field vector descriptions given above
are best at lower latitudes, including all latitudes reached by
Voyager during the Saturn encounter]
Ancillary Data
==============
An ancillary data file containing the derived spacecraft magnetic
field is provided with this data at 48 second sample rate. These
data provide zero level offsets used in the data processing.
Spacecraft field data is provided in spacecraft coordinates as
follows:
--------------------------------------------------------------------
48 Second Spacecraft Field (Payload) Averages
--------------------------------------------------------------------
Column Type Description
--------------------------------------------------------------------
time a23 spacecraft event time (UT) of the sample in the
format: yyyy-mm-ddThh:mm:ss.sss
sclk a12 spacecraft clock in the format:
MOD65536:MOD60:FDS-LINE
mag_id i1 magnetometer ID (1 = LFM, 2 = HFM)
Bx 1pe10.3 average Bx spacecraft field in nT (payload coords.)
By 1pe10.3 average By spacecraft field in nT (payload coords.)
Bz 1pe10.3 average Bz spacecraft field in nT (payload coords.)
Bmag 1pe10.3 magnitude of spacecraft field in nT
rms_Bx 1pe10.3 root-mean-square deviation of Bx over the averaging
interval in nT
rms_By 1pe10.3 root-mean-square deviation of By over the averaging
interval in nT
rms_Bz 1pe10.3 root-mean-square deviation of Bz over the averaging
interval in nT
npts i4 number of points in the average
Coordinate Systems
==================
These data are provided in a heliographic right handed spherical
coordinate system known as RTN. RTN coordinates are defined by the
following directions:
R - radial direction: along the Sun to spacecraft line, positive
in the anti-Sunward direction
T - tangetial direction: Omega (the Sun's spin axis) X R (parallel
to the Solar Equatorial plane)
N - normal direction: completes the right handed set
References
==========
The reference documents for this data set are:
[ACUNAETAL1981B] Acuna, M.H., J.E.P. Connerney, and N.F. Ness,
Topology of Saturn's Main Magnetic Field,
Nature, 292, 721, 1981.
[BEHANNONETAL1981B] Behannon, K.W., J.E.P. Connerney, and N.F.
Ness, Saturn's Magnetic Tail: Structure and
Dynamics, Nature, 292, 753, 1981.
[BEHANNONETAL1983] Behannon, K.W., R.P. Lepping, and N.F. Ness,
Structure and Dynamics of Saturn's Outer
Magnetosphere and Boundary Regions, J.
Geophys. Res., 88, 8791, 1983.
[CONNERNEYETAL1983] Connerney, J.E.P., M.H. Acuna, and N.F. Ness,
Currents in Saturn's Magnetosphere, J.
Geophys. Res., 88, 8779, 1983.
[LEPPINGETAL1981D] Lepping, R.P., L.F. Burlaga, and L.W. Klein,
Surface Waves on Saturn's Magnetopause,
Nature, 292, 750, 1981.
[NESSETAL1981] Ness, N.F., M.H. Acuna, R.P. Lepping, J.E.P.
Connerney, K.W. Behannon, L.F. Burlaga, and
F.M. Neubauer, Magnetic Field Studies by
Voyager 1: Preliminary Results at Saturn,
Science, 212, 211, 1981.
[NESSETAL1982B] Ness, N.F., M.H. Acuna, K.W. Behannon, L.F.
Burlaga, J.E.P. Connerney, R.P. Lepping, and
F.M. Neubauer, Magnetic Field Studies by
Voyager 2: Preliminary Results at Saturn,
Science, 215, 558, 1982.
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