Lunar Prospector Electron Reflectometer Derived Bundle
Lunar Prospector Electron Reflectometer Electron Reflection Data Description
PDS3 DATA_SET_ID = LP-L-ER-4-ELECTRON-DATA-V1.0
ORIGINAL DATA_SET_NAME = LP MOON ER LEVEL 4 ELECTRON DATA V1.0
START_TIME = 1998-06-08
STOP_TIME = 1999-06-27
PDS3 DATA_SET_RELEASE_DATE = 2003-10-30
PRODUCER_FULL_NAME = DR. DAVID MITCHELL
References:
========
Binder, A.B., W.C. Feldman, G.S. Hubbard, A.S. Konopliv, R.P. Lin, M.H. Acuna,
and L.L. Hood, Lunar Prospector searches for polar ice, a metallic core, gas
release events, and the moon's origin, Eos, Trans. AGU, 79, 97, 1998.
(https://doi.org/10.1029/98EO00061)
Acuna, M.H. J. Connerney, P. Wasilewski, R. Lin, K. Anderson, C. Carlson,
J. McFadden, D. Curtis, R. Reme, A. Cros, J. Medale, J. Sauvaud, C. d'Uston,
S. Bauer, P. Cloutier, M. Mayhew, and N. Ness, Mars Observer magnetic fields
investigation, J. Geophys. Res., 97, 7799-7814, 1992.
(https://doi.org/10.1029/92JE00344)
Carlson, C., D. Curtis,G. Paschmann, and W. Michael, An instrument for rapidly
measuring plasma distribution functions with high resolution, Adv. Space Res.,
2, 67, 1983.
(https://doi.org/10.1016/0273-1177(82)90151-X)
DATA_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 collection was prepared from the ER Low-Resolution (ERLR) data
set. The following description is an overview of the processing of
the ERLR collection, adapted from PDS documentation.
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
CONFIDENCE_LEVEL_NOTE =
Review:
======
These data have completed peer review and are certified.
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.
|