Lunar Prospector Magnetometer Derived Bundle
Lunar Prospector Large-Scale Vector Field Maps Data Description
PDS3 DATA_SET_ID = LP-L-MAG-5-SURFACE-FIELD-MAP-V1.0
ORIGINAL DATA_SET_NAME = LP MOON MAG LEVEL 5 SURFACE MAGNETIC FIELD
MAPS V1.0
START_TIME = 1998-01-16
STOP_TIME = 1999-07-29
PDS3 DATA_SET_RELEASE_DATE = 2003-10-01
PRODUCER_FULL_NAME = LON L. HOOD
Overview:
=========
Lunar Prospector Magnetometer (MAG) (NASA Level 4 CODMAC Level 5) Derived
Data. Large-Scale Vector Field Maps at a Common Altitude of 37 km.
The spatial resolution of the grid (0.25 x 0.25 degrees) is much
less than the mean S/C altitude.
====================================================================
Sampling:
=========
The measured parameters consist of the three vector components
of the lunar crustal magnetic field in East, North, and Radial (ENR)
coordinates, and the field magnitude at an altitude of 37 km. The
units of the field measurements are nanoTeslas (nT). 1 nT = 0.00001
Gauss. ENR coordinates are centered at the instantaneous spacecraft
location with X directed eastward, Y directed northward, and Z directed
radial to the Moon.
The constant spacecraft altitude is estimated by subtracting the mean
lunar radius (1738 km) from the radial distance of the spacecraft
to the lunar center of figure.
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Processing:
===========
Step 1: The Level 3 MAG data (two-dimensional, filtered regional field
maps at the spacecraft altitude) are continued to a common altitude.
Only those maps produced at relatively low altitudes (less than 50 km)
during the extended mapping phase of the LP mission are processed to
obtain maximum accuracy and horizontal resolution. Only upward
continuation is applied in order to prevent amplification of data
errors by the continuation procedure. The upward continuation was
accomplished by applying a simple power law algorithm to each field
component on the regional maps. The exponent for the power law was
determined empirically by comparing measured field components at
several different altitudes over the same regions. The field magnitude
is calculated from the smoothed component files on the same
two-dimensional grid. The resulting two-dimensionally filtered
gridded files at a common altitude are the Level 4 data. They
contain latitude, longitude, the field component in nT (or the
altitude in km), and the number of data points in each bin.
Step 2: If a significant longitudinal gap between
usable orbit segments is present, linear interpolation between the
two nearest orbit segments is applied to create an interpolated
orbit segment. This interpolated segment is then read into the
two-dimensional filtering code to fill in the missing grid values.
This method is applied only when the gap between usable orbits is
smaller than the 5-bin size of the two-dimensional filter. Where
possible, the resulting regional gridded files at a common altitude
are joined together to construct larger-scale gridded files.
Step 3: For visualization purposes, a suitable
graphics utility is applied to construct regional shaded contour plots
of the vector components of the crustal field and the field magnitude.
These are contained in .tif files that are included with the filtered
gridded files in appropriate subdirectories identifying their location
on the Moon. The .tif files are labeled in the format east.tif,
north.tif, radial.tif, and total.tif. The contour interval on all
field plots isfield plots is 2 nT. (Note: On the PDS disk volume,
the graphics files are JPEG files and are under the BROWSE directory.)
====================================================================
Format:
=============
File naming convention: B_aaa-bbbc_ddd.TAB, where:
aaa and bbb indicate the initial and final latitude or
longitude for the file;
c is the letter N, S, E or W, indicating a compass direction
(this also tells you whether the aaa and bbb are latitude
or longitude; for example, if c is E, then aaa-bbb must
be east longitude);
ddd indicates which field component is mapped in the file
(EAS = East field component; NOR = North field component;
RAD = Radial field component; TOT = total field).
File name example: B_120-180E_RAD.TAB describes the radial
component of B from 120-180 east longitude.
Data File Description:
Column 1: Latitude at this grid point (South is negative; North
is positive).
Column 2: East longitude at this grid point.
Column 3: Magnetic field value in nanoTeslas at constant altitude
of 37 km. The extension of the file name indicates what
is contained in this column (EAS = East field component;
NOR = North field component; RAD = Radial field component;
TOT = total field)
Column 4: Number of data points in this latitude/longitude bin.
The bin size can be deduced from the difference between
successive latitude and longitude values (0.25 degrees
by 0.25 degrees for this altitude).
======================================================================
Coordinate Systems:
===================
The coordinate system used to define file coverage, as described in
the Media/Format section, is selenographic.
======================================================================
Ancillary Data:
===============
No ancillary data are provided with this dataset.
======================================================================
Software:
=========
No software is provided with these data.
CONFIDENCE_LEVEL_NOTE =
======================================================================
Review:
=======
These data have completed peer review and are certified.
======================================================================
Limitations:
============
The quality of the measurements is a strong function of the
spacecraft altitude, which varies from a minimum of about 80 km
during the first year of the mission to a minimum of about 15 km during
the final six months of the mission. The best crustal field measurements
were therefore obtained during the final ``extended mission'' phase
when the orbit periapsis was lowest. The surface resolution of the
measurements is normally of the same order as the spacecraft altitude.
At the lowest altitudes (about 15 km), the resolution is limited also
by the orbit track separation, which is approximately 30 km at the lunar
equator. The accuracy to which the crustal field is measured at a single
point along the spacecraft trajectory is limited by residual external
field variations and is approximately 0.1 nT. The upward continued
field maps at 37 km altitude have essentially the same horizontal
resolution and accuracy as the Level 3 regional maps (about 25 km and
0.1 nT, respectively).
======================================================================
Data Coverage:
==============
The coverage of the measurements over the lunar surface is limited to
those areas on the near and far side when the Moon and the LP spacecraft
were favorably located to allow direct measurements of the crustal
magnetic field.
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