: Overview: : Lunar Prospector magnetometer (MAG) Level 4 Data (CODMAC Level 5). 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. : 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.) : Media/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.

: Review: : These data have been examined by the data producer, and as of 2003-10-01 are being prepared for peer review by the PDS. : 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.