Overview:
========

Magnetometer data records are time-ordered series of magnetic vector
measurements. Each record consists of a time tag followed by six
scalar values representing the magnetic field vector, measured in
nanoteslas, in two different coordinate systems: selenocentric solar
ecliptic (SSE) and body-fixed selenographic (SEL), followed by the rms
deviation of the field magnitude, which is independent of the
coordinate system.  The spacecraft position is given in both of the
above coordinate systems.  These data are obtained continuously at 9
Hz and are averaged in 5-second intervals for this data archive.


Parameters:
==========

Magnetic field data are provided in units of nanotesla (nT).


Processing:
==========

Magnetic field data are sampled onboard at 18 Hz and averaged to 9 Hz
before being placed into telemetry. In order to cover a very large
dynamic range with 12-bit values, the full range of the instrument is
divided into 8 sub-ranges. Range changing is performed dynamically
onboard based on the ambient field strength. The first step in the
processing is to extract the data from telemetry and form time-tagged
magnetic field vectors.  Then, occasional data spikes due to range
changes or bad telemetry are flagged; these data points are not used in
the offset determination.  The instrumental offsets for Bx and By are
calculated simultaneously for 1 minute data windows using a technique
developed by M. Acuna.  Time varying offsets are needed because the
offsets drift slightly as the magnetometer temperature changes. (The
MAG temperature is modulated as the spacecraft goes in and out of the
Moon's shadow.) A constant offset is used for the Z component, because
lack of spin modulation in that component precludes routine offset
determination. Offsets are calculated separately for each data range
and subtracted from the data. This correction reduces systematic errors
in the X and Y components to less than ~0.1 nT. Since the Z component
cannot be corrected in this way, systematic errors in that axis (due to
temperature-induced offset drifts) can be as large as 0.5 nT.
Instrumental gains, different for each of the 8 ranges, are then
applied to convert to nanotesla. Next, the data are corrected for a
slight misalignment between the magnetometer sensor axes and the
spacecraft axes. The resulting sensor (SEN) coordinate system has its
Z-axis parallel to the spacecraft spin axis, and its X-axis aligned
with the magnetometer boom. The SEN coordinate system rotates as the
spacecraft spins. The next step is to flag spurious data values. The
first measurement following a range change is flagged, since the finite
time needed to make the change often corrupts the first measurement in
the new range. Other false spikes also appear in the data, most of
which are attributable to occasional noise in the telemetry. A
comparison technique is used to remove outliers.  In practice, this
effectively removes most  spurious data values without eliminating any
valid data. The next step is to ''despin'' the data from SEN to
''despun spacecraft'' (SCD) coordinates, which are defined such that
the Z-axis is parallel to the spacecraft spin vector, and the direction
of the sun is in the half-plane defined by X > 0, Y = 0. Despinning is
performed using the reconstructed sunpulse data, which are corrected
for spacecraft spin-up in the Moon's shadow.  Next, the data are
averaged in 5-second intervals; this reduces the data volume by a
factor of 45.  Finally, a rotation is performed from SCD coordinates to
selenocentric solar ecliptic (SSE) and body-fixed selenocentric (SEL)
coordinates using the spacecraft ephemeris data (the latitude and
longitude of the spin axis obtained from files included in the PDS
distribution of the LP Level-1 magnetic field data) and lunar ephemeris
data obtained from the Jet Propulsion Laboratory's Horizons system
(). SSE coordinates are defined
such that the X-axis points from the center of the Moon to the center
of the Sun, the Z-axis is parallel to Earth's ecliptic north, and the
Y-axis completes the right-handed coordinate system. SEL coordinates
are defined such that the Z-axis is parallel to the Moon's spin vector
(north pole) and the X and Y axes intersect the lunar equator. The
X-axis intersects the lunar equator at 0 degrees longitude, and is thus
nearly aligned with the Moon-Earth line. (It is not exactly aligned
because of the Moon's libration.) The Y-axis intersects the lunar
equator at 90 degrees EAST longitude: SEL coordinates are right handed.


Media/Format:
============

Data are archived on CDROMs in level 1 compliance with the ISO 9660
standard. Three CDROMs cover the entire mission. The data are provided
as ASCII ''tables'' of 1-day duration in Selenocentric Solar Ecliptic
(SSE) and Selenographic (SEL) coordinates.  Date/time are given in 2
formats as described below.

MAG Data:
Naming convention: MAyymmdd.TAB

Parameters:

1) time parameter 1:  PDS date-time format of the mid-time of the 5-sec
averaging window in spacecraft event time, i.e., Universal Time at the
spacecraft.  Example: 1998-11-08T05:50:42.5

2) time parameter 2:  decimal day of the mid-time of the 5-sec averaging
window in spacecraft event time, i.e., Universal Time at the spacecraft.

3) mag_field_SEL:  Array[3] giving B-field components (nT) in the SEL
coordinate system

4) mag_field_SSE: Array[3] giving B-field components (nT) in the SSE
coordinate system

5) mag_field_RMS: RMS deviation (nT) of the field magnitude.  Provides
an indication of field variability for the 5-sec window.

6) Spacecraft SEL coordinates: coordinate array[3]  (km)

7) Spacecraft SSE coordinates: coordinate array[3] (km)

8) ISUN - a parameter from the sunpulse file indicating whether the
spacecraft is in the sun (0), in eclipse (1), or, if the sunpulse file
was not available, the data were processed using the less accurate
determination of the sunpulse time in the Level-0 data file and ISUN is
set to 2.  Note that this parameter contains erroneous values (0 <-> 1)
from time to time.  To reduce the occurrence of bad ISUM values the
parameter was median-filtered with a 9-point window, which removes most
of the errors.

These parameters could be named PDS_time, decimal_day, Bx_sel, By_sel,
Bz_sel, Bx_sse, By_sse, Bz_sse, B_rms, x_sel, y_sel, z_sel, x_sse,
y_sse, z_sse, isun

An appropriate format for reading the data is:

format='( A21, f12.6, f9.3, 2(f8.3), f9.3, 2(f8.3), f9.3, f10.2,
2(f9.2), f10.2, 2(f9.2), I3 )'

however, the records contain blanks between each parameters so that a
format statement will not be required by most languages.


Ancillary Data:
==============

There are several ancillary data files provided with this archive.
These include:

Spacecraft Attitude data LP-L-ENG-6-ATTITUDE-V1.0
Spacecraft Ephemeris data LP-L-6-EPHEMERIS-V1.0
Spacecraft Position data LP-L-6-POSITION-V1.0
Spacecraft Command logs LP-L-ENG-6-COMMAND-V1.0

These data sets provide additional information about the state of the
spacecraft and the instrument during data acquisition that may aid in
the scientific analysis of this data set.


Coordinate Systems:
==================

The SSE coordinate system has its X-axis along the Moon-Sun line,
positive towards the Sun. The Z-axis is parallel to the northward
normal to the Earth's ecliptic plane, and Y completes the right-handed
set. The SEL coordinate system used here is a Cartesian representation
that places the Z-axis along the rotation axis of the moon, positive in
the direction of angular momentum. The X-axis lies in the lunar
equatorial plane at 0 degrees longitude, and Y completes the
right-handed set.


Software:
========

There are no software provided with this data archive.

Review:
======

Magnetometer data quality was reviewed by D.L. Mitchell (UC-Berkeley),
M.H. Acuna (NASA-GSFC), and L.L. Hood (LPL-Univ. Arizona). Further
review was conducted by R.J. MacDowall (NASA-GSFC), E. Guandique
(NASA-GSFC-Emergent), M. Kaelberer (NASA-GSFC-Emergent), and M.H. Acuna
(NASA-GSFC).


Limitations:
===========

The magnetometer data should be used in conjunction with spacecraft
ephemeris data so that perturbations to the ambient magnetic field
vector due to crustal sources can be localized in selenographic
coordinates. It is also important to evaluate the plasma environment
using electron reflectometer data, since the Moon can be in the solar
wind, in the Earth's magnetosheath, in a geomagnetic tail lobe, or in
the geotail current sheet. The plasma environment can strongly
influence the usefulness of the data for probing lunar crustal magnetic
fields. By far, the best (steadiest) data are obtained in the
geomagnetic tail lobes.


Data Quality:
============

Magnetometer data are of excellent quality. Systematic errors in the
magnetic field component orthogonal to the spacecraft spin vector have
been reduced to ~0.1 nT by time-variable offset corrections (see
above). Systematic errors in the magnetic field component parallel to
the spin axis can be as large as ~0.5 nT.  The B_RMS value provides an
indication of the field variability during the 5-sec window.  A single,
large value for the B_RMS would likely indicate that bad-telemetry or
some similar problem had caused large variability during a single
averaging window.


Data Coverage:
=============

Magnetic field data are obtained continuously; however, telemetry gaps
do occur. A table of gaps in the raw merged telemetry data OUTAGES.TAB)
is available in the Level 0 Lunar Prospector archive, but is not part
of the present Level 1 archive. Other gaps may exist due to data
contamination or processing limitations.  In particular, we list in
Appendix A, times when we have explicitly removed data from the dataset
for reasons such as corrupted sun pulse data preventing the despinning
of the magnetic field data.   There are also a number of data gaps that
result because the MAG/ER instruments were telemetering burst mode
data; these intervals are listed in Appendix B.


APPENDIX A - Time intervals for which Magnetic field data were deleted
from this archive

Year    Month   Day     Start decday    Stop decday
1998      4      8       98.407899       98.412240
1998      5      1      121.704774      121.733883
1998      5     15      135.739902      135.743721
1998      5     16      136.570399      136.586314
1998      8     15      227.220689      227.232726
1998      8     17      229.684809      229.712471
1998     12      3      337.314265      337.314612
1999      1     29       29.313513       29.338455
1999      2     25       56.501591       56.521962
1999      3      3       62.877170       62.954948
1999      3     24       83.758767       83.782089
1999      4     16      106.511603      106.515480
1999      4     17      107.210503      107.211429
1999      5      1      121.045168      121.080874
1999      5     12      132.137703      132.149334


APPENDIX B - Approximate times of data gaps due to MAG/ER burst mode
telemetry

Year  Month  Day  Day of year: time of day
1998   12    03   day 337: 3 intervals from 5:20-10:15
1999   03    03   day  62: 21:00-24:00
1999   03    04   day  63:  0:00- 0:35
1999   03    18   day  77: 3 intervals from 11:30-24:00
1999   03    19   day  78: 4 intervals from  0:00-24:00
1999   03    20   day  79: 4 intervals from  0:00-24:00
1999   03    21   day  80: 4 intervals from  0:00-13:30
1999   04    14   day 104: 22:30-24:00
1999   04    15   day 105: 4 intervals from  0:00-24:00
1999   04    16   day 106: 5 intervals from  0:00-24:00
1999   04    17   day 107: 5 intervals from  0:00-24:00
1999   06    08   day 159: 2 intervals from 15:20-24:00
1999   06    09   day 160: 4 intervals from  0:00-24:00
1999   06    10   day 161: 5 intervals from  0:00-24:00
1999   06    11   day 162: 2 intervals from  0:00-17:30
1999   07    05   day 186: 2 intervals from 16:00-24:00
1999   07    06   day 187: 4 intervals from  0:00-24:00
1999   07    07   day 188: 5 intervals from  0:00-24:00
1999   07    08   day 189: 4 intervals from  0:00-18:00