Data Set Information
DATA_SET_NAME LP MOON ER LEVEL 4 ELECTRON DATA V1.0
DATA_SET_ID LP-L-ER-4-ELECTRON-DATA-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION : Overview: : Lunar Prospector Electron Reflectometer (ER) Level 2 Data represent a time ordered series of derived quantities from electron reflection measurements by the Electron Reflectometer (ER) instrument aboard the Lunar Prospector (LP) polar orbital mission to the Moon (January 1998 to July 1999). Each data file contains data from a single energy channel: 200 eV, 220 eV, 340 eV, 520 eV, or 590 eV. The processing level of these data is Level 2 by NASA standards, but is Level 4 according to the CODMAC definitions. : Parameters: : Each record consists of a time tag followed by 7 scalar values. The first two columns after the time tag provide the selenographic (body-fixed) longitude and latitude of the ER measurement footprint, which is obtained by extrapolating the magnetic field vector measured at the spacecraft along a straight line until it intersects the Moon. The next column gives the average magnetic field amplitude in nanoteslas, measured at the spacecraft (|B_sc|), during the time interval of each ER measurement. The next two columns give the cutoff pitch angle (A) and its uncertainty (in degrees) for electrons reflected from the lunar surface at the energy channel of the file. The last two columns give the effective electron reflection coefficient (R) and its uncertainty. The effective reflection coefficient is the ratio of the reflected flux to the incident flux for an ideal uniform pitch angle distribution, and is an indicator of surface magnetic field strength. This is calculated from the loss cone angle: R : |cos(A)|. From this, an estimate of the surface magnetic field, uncorrected for electrostatic reflection, can be calculated: |B_surf| > |B_sc|*[R^2/(1 - R^2)]. : Processing: : This data set was prepared from the ER Low-Resolution (ERLR) data set. The following description is an overview of the processing of the ERLR data set, adapted from PDS documentation for that data set. Processing is carried out at the Space Sciences Laboratory (SSL) of the University of California, Berkeley (UCB), to convert the raw data to measurements of the omnidirectional electron flux (cm-2 s-1 ster-1 eV-1) as a function of time. Because of the instrument's high dynamic range (six decades), the onboard digital processing unit (DPU) compresses the raw counts in a logarithmic scale. The first step is to decompress the raw counts and construct a two-dimensional data array, where the first dimension is time (1 element every 16 spins), and the second dimension is energy (15 elements). Raw count rate (R) is obtained by dividing the raw counts by the integration time, which is a function of energy. In general terms, the integration time is longer at higher energies in order to improve counting statistics. The data are next corrected for deadtime. During the time it takes the instrument to process a single electron (known as the ''deadtime'', which is about 0.3 microsec for the ER), it ignores any other electrons. The raw count rate is multiplied by the factor 1/(1 - RT), where T is the deadtime, to obtain the corrected count rate. Data values are masked when the deadtime correction factor exceeds 1.25. Note that a background count rate due to cosmic rays and noise in the electronics (about 10 counts/sec) has not been subtracted. In most cases, measurements in the highest energy channel (20 keV) are dominated by background, which allows this channel to be used as a baseline for estimating the background level in lower energy channels. Finally, one divides by the geometric factor (0.02 cm2 ster) and the center energy (eV) to obtain the differential particle flux (cm-2 s-1 ster-1 eV-1). Electron flux uncertainties include Poisson counting statistics and digitization noise (associated with the lossy logarithmic compression used to maximize science return within the ER telemetry allotment). Flux uncertainties DO NOT include the absolute uncertainty in the geometric factor, which was estimated from electrostatic optics simulations, including corrections for internal grid transmissions and MCP efficiency. However, absolute calibration is not necessary for most applications of these data, which are based on the shape of the pitch angle distribution and not its absolute flux level. The differential particle flux is measured in 16 angular sectors spanning the 360-degree disk-shaped field of view. During one half of a spacecraft spin (~2.5 seconds) this field of view sweeps over the entire sky (4-pi steradians). Given the magnetic field measured onboard by the LP Magnetometer, the field of view is mapped into pitch angle (the angle between the electron velocity and the magnetic field direction) to create a pitch angle distribution. A step function is fit to the pitch angle distribution to determine the cutoff pitch angle (A) and its associated statistical uncertainty. : File Names and Format: : Each file (in ASCII format) is named as yyyymm.TAB, where yyyy is the year (1998 or 1999), mm is the month (01 through 12), and 'TAB' indicates an ASCII table file. The PDS archive contains one subdirectory of such files for each energy channel. Each record begins with the Universal date (yyyy-mm-dd) and time (hh:mm:ss) of the record, separated by a slash character. This is followed by the selenographic (body-fixed) longitude and latitude, the average magnetic field amplitude at the spacecraft (in nanoteslas), the cutoff pitch angle for reflected electrons, the uncertainty in the cutoff angle, the effective electron reflection coefficient (R), and the uncertainty in R. List of columns in the files: Column 1: Universal date and time Column 2: Selenographic longitude Column 3: Selenographic latitude Column 4: Average magnetic field amplitude at spacecraft Column 5: Cutoff pitch angle Column 6: Uncertainty in cutoff pitch angle Column 7: Effective electron reflection coefficient Column 8: Uncertainty in effective electron reflection coefficient
DATA_SET_RELEASE_DATE 2003-10-30T00:00:00.000Z
START_TIME 1998-06-08T12:00:00.000Z
STOP_TIME 1999-06-27T12:00:00.000Z
MISSION_NAME LUNAR PROSPECTOR
MISSION_START_DATE 1995-01-01T12:00:00.000Z
MISSION_STOP_DATE 1999-07-31T12:00:00.000Z
TARGET_NAME MOON
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID LP
INSTRUMENT_NAME ELECTRON REFLECTOMETER
INSTRUMENT_ID ER
INSTRUMENT_TYPE ELECTRON REFLECTOMETER
NODE_NAME Planetary Plasma Interactions
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Review: : ER data quality has been reviewed at U.C. Berkeley by D. Mitchell, J. Halekas, M. Oieroset, T. Phan, S. Frey, and D. Curtis. Limitations: : The ER Level 2 data are intended to be used in conjunction with magnetic field and spacecraft ephemeris data. Electrons travel along the magnetic field lines in tight helices (few km radius) at high speed (order of one Moon diameter per second). Thus the electron data contain information about the plasma environment as well as the large-scale configuration of the magnetic field, which is sampled locally by the MAG. Data Quality: : The ER data are generally of very high quality. Three instrumental effects should be noted. (1) Sunlight directly enters the ER aperture twice per spacecraft spin. These photons scatter within the instrument and produce secondary electrons, which cause spurious counts. These counts have not been removed. (2) Electron fluxes are relatively high at low energies, and at times the instrument becomes saturated. During processing, a deadtime correction of the form 1/(1 - RT) is applied, where R is the measured count rate and T is the time needed to analyze a single electron. This correction is only reliable up to values of about 1.25. (3) Low energy electrons can be perturbed by the spacecraft floating potential relative to the plasma in which the spacecraft is immersed. In sunlight, the spacecraft floats a few volts positive, and in the Moon's shadow, it floats tens of volts negative. Electrons must cross this potential before they enter the ER electrostatic optics; thus, all electron energies are shifted by this potential relative to their energies far from the spacecraft. No corrections are made for spacecraft potential effects. Data Coverage: : ERLR data are obtained continuously; however, gaps occur due to telemetry interruptions, data contamination or processing limitations.
CITATION_DESCRIPTION Mitchell, D., LP-L-ER-4-ELECTRON-DATA-V1.0, LP MOON ER LEVEL 4 ELECTRON DATA V1.0, NASA Planetary Data System, 2003.
ABSTRACT_TEXT Lunar Prospector Electron Reflectometer (ER) Level 2 Data (CODMAC Level 4). Time ordered values of derived quantities from electron reflection, including calculated effective electron reflection coefficient.
PRODUCER_FULL_NAME DR. DAVID MITCHELL
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