Lunar Prospector Electron Reflectometer Calibrated Bundle
Lunar Prospector Electron Reflectometer 3-D Flux Data Description
PDS3 DATA_SET_ID = LP-L-ER-3-RDR-3DELEFLUX-80SEC-V1.1
ORIGINAL_DATA_SET_NAME = LP ELECTRON REFLECTOMETER 3D ENERGY SPECTRA
80SEC V1.0
START_TIME = 1998-01-16
STOP_TIME = 1999-07-29
PDS3 DATA_SET_RELEASE_DATE = 1999-01-01
PRODUCER_FULL_NAME = DR. DAVID MITCHELL
Data Description
================
Overview
========
The Electron Reflectometer 3-D (ER3D) data is a time ordered series
of electron measurements from the Lunar Prospector (LP) Mission.
Each record consists of a time tag with an 88 x 15 array of scalar
data points representing measurements of the electron flux in 88
solid angles covering 4-pi steradians and 15 energy channels
logarithmically spaced from 10 eV to 20 keV, with an energy
resolution (dE/E) of 25%. Each data point is a measure of the
electron flux (cm-2 sec-1 ster-1 eV-1) within one of the 88
approximately equal-sized solid angles. Angles are given in ''despun
spacecraft'' (SCD) coordinates, in which the Z-axis coincides with
the spacecraft spin vector, and the direction of the sun is in the
half plane defined by X ❯ 0, Y = 0. These data records are
accumulated during 1/2 spin of the spacecraft (~2.5 seconds) and
sampled every 16 spins (~80 seconds). For convenience, the average
magnetic field vector during the ~2.5-second accumulation time is
provided to facilitate calculation of electron pitch angle
distributions from the 3-D data.
Parameters
==========
The ER 3-D data are provided as differential electron flux in units
of particles per square cm per second per steradian per electron
volt. The center angles of the 88 solid angle bins are provided in
degrees of longitude and latitude in the SCD coordinate system (see
below for the definition of SCD coordinates). Note that the center
angles vary with energy, since the spacecraft rotates by 11.25
degrees during the time that the ER sweeps through its energy range.
Each of the 88 solid angle bins are approximately the same size. The
exact sizes are given in the following table:
Latitude Range Number of Solid Angle
(degrees) Longitude Bins Bin Size (ster)
-----------------------------------------------------
67.5 to 90.0 4 0.119570
45.0 to 67.5 8 0.170253
22.5 to 45.0 16 0.127401
0.0 to 22.5 16 0.150279
0.0 to -22.5 16 0.150279
-22.5 to -45.0 16 0.127401
-45.0 to -67.5 8 0.170253
-67.5 to -90.0 4 0.119570
-----------------------------------------------------
In the above table, latitudes are given in SCD coordinates, and the
longitude bins are of equal size within each latitude range.
ER 3-D distributions are recorded for 15 different energies ranging
from 10 eV to 20 keV. The energy sweep was changed several times
during the mission for both instrumental and scientific reasons.
There are two basic sweeps, the first from 40 eV to 20 keV, and the
second from 7 eV to 20 keV. The first is used to avoid instrumental
saturation in the solar wind, where fluxes are relatively high. The
second is used to enhance the science return in the geomagnetic tail
lobes, where fluxes are relatively low.
Processing
==========
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 electron flux (cm-2 s-1 ster-1 eV-1) in 88
solid angles 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
three-dimensional data array, where the first dimension is time (1
element every 16 spins), the second dimension is direction (88
elements spanning 4-pi steradians), and the third 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 and solid angle. In
general terms, the integration time is longer at higher energies (to
improve counting statistics) and near the poles of the SCD coordinate
system, because the ER's disk-shaped field of view (FOV) always
includes the poles but requires the spacecraft spin to sweep the FOV
along the equator. 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).
The electron directions (i.e., the directions in the centers of each
of the 88 solid angles) are rotated into despun spacecraft (SCD)
coordinates using sunpulse data. Special care must be taken when the
spacecraft is in the Moon's shadow, where the spacecraft spins up
slightly as the booms cool and contract. This gradual spin-up is
reconstructed on the ground and is not available in real time as the
ER is collecting data and sorting it into the 3-D data array. Thus,
the 3-D look directions must be corrected during post processing
based on the reconstructed sunpulse data. Note that an electron's
direction is parallel to its velocity. In other words, we record
where the electrons are going to, not where they are coming from.
Finally, the average magnetic field direction (in SCD coordinates)
during the accumulation time of the 3-D electron distribution is
calculated and saved with each 3-D data record.
Format
======
The data are provided as ASCII tables of 1 day duration. Each record
contains data sampled over 16 spacecraft spins (but accumulated over
each 1/2 spacecraft spin) so that a full 3-D spectrum can be produced as
the detector spins and steps. To satisfy telemetry constraints, full
3-D spectra are obtained once every 16 spacecraft spins.
A row (record) in a data file consists of the following fields:
PDS_time: Time in PDS standard time format.
time: Time in seconds since Jan. 1, 1970.
energy: Energy bin value.
spec_no: Spectrum number (see below under
'Table structure').
MagFieldDespunSCCoords: Array[3] giving magnetic field values.
ele_flux: Array[88] giving electron flux in 88 angle bins.
dist_phi: Array[88] giving phi values.
Record format:
PDS_time (A19), time (1X, F15.0), energy (1X, F9.3),
spec_no (1X, I4, 1X), MagFieldDespunSCCoords 3(F13.6, 1X),
ele_flux 88(1X, E9.3), phi 88(1X, F6.2).
Each record is terminated by the carriage return and line feed
(CR/LF) characters.
Table structure:
Each spectrum of 15 electron energies is represented by a set of
15 consecutive records in the table. Each such spectrum has a
distinct spectrum number, which occurs in each record in the
15-record set.
In addition, there is a file THETA.TAB that gives the
inclination angle (theta) for each detector step as the
instrument cycles. These angles are fixed. There is one theta
value associated with each of the 88 electron flux values.
THETA.TAB is an ASCII table with one row and 88 columns.
The data were placed in their present format after processing.
Ancillary Data
==============
There are several ancillary data files provided with this
archive. These include:
Spacecraft Attitude data:
Binder, A.B., Feldman, W.C., Konopliv, A.S., Lin, R.P., Acuna, M.H.,
Hood, L.L., and Guinness, E.A., LP MOON SPACECRAFT ATTITUDE V1.0,
LP-L-ENG-6-ATTITUDE-V1.0, NASA Planetary Data System, 1998.
Spacecraft Ephemeris data:
Binder, A.B., W.C. Feldman, A.S. Konopliv, R.P. Lin, M.H. Acuna,
L.L. Hood, and E.A. Guinness, LP MOON SPACECRAFT EPHEMERIS V1.0,
LP-L-6-EPHEMERIS-V1.0, NASA Planetary Data System, 1998.
Spacecraft Position data:
urn:nasa:pds:lp-mager-derived:data-spacecraft-pos
Spacecraft Command logs:
Binder, A.B., Feldman, W.C., Konopliv, A.S., Lin, R.P., Acuna, M.H.,
Hood, L.L. and Guinness, E.A., LP MOON UPLINK COMMAND V1.0,
LP-L-ENG-6-COMMAND-V1.0, NASA Planetary Data System, 1998.
These collections provide additional information about the state
of the spacecraft and the instrument during data acquisition
that may aid in the scientific analysis of this collection.
Coordinate System(s)
====================
The 3-D data are provided in 'despun spacecraft' (SCD)
coordinates, in which the Z-axis coincides with the spacecraft spin
vector, and the direction of the sun is in the half plane defined
by X ❯ 0, Y = 0. Throughout most of the mission the spacecraft
spin vector was within a few degrees of either the north or south
ecliptic pole. Thus, SCD coordinates are closely related to GSE
coordinates. Conversion to GSE coordinates is not performed because
the electrons are constrained to move along the magnetic field lines
in helices with radii of only a few kilometers. Thus, only electron
directions with respect to the magnetic field are meaningful. The
magnetic field direction is provided in SCD coordinates for each ER
3-D distribution, so that electron pitch angle distributions can be
readily calculated.
Software
========
There are no software provided with this data archive.
Review:
======
These data have completed peer review and are certified.
Limitations
===========
The ER 3-D data are intended to be used in conjunction with
magnetic field and spacecraft ephemeris data, from which
electron pitch angle distributions can be calculated and
magnetic field line tracing to the lunar surface can be
performed. Magnetic field data are preprocessed (rotated to SCD
coordinates and averaged over the 3-D integration time) and
provided with each 3-D data record to facilitate calculation of
pitch angle distributions. The magnetic field data are archived
in separate tables in two different coordinate systems, which,
together with spacecraft ephemeris data, allows field line
tracing to the lunar surface.
Data Quality
============
The ER 3-D 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 a false signal that can overwhelm the desired signal
from ambient electrons. In the SCD coordinate system, sunlight
contamination appears as a flux spike in the direction of the
sun (0 degrees longitude, ~0 degrees latitude). The flux spike
has a longitude width of about 14 degrees, and is more elongated
in latitude. (2) Electron fluxes are relatively high at low
energies, and at times the instrument becomes saturated.
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, above which data are flagged. (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. In addition,
the trajectories of low energy electrons can be bent by the
potential, so the imaging characteristics of the ER are degraded
for electron energies that are comparable to or lower than the
spacecraft potential. No corrections are made for spacecraft
potential effects.
Data Coverage
=============
ER 3-D data are obtained during 1/2 spin (~2.5 seconds) and
sampled every 16 spins (~80 seconds). During this time, the
spacecraft moves about 128 km in its orbit.
A table of gaps in the raw merged telemetry data (OUTAGES.TAB) is
included in the Level 0 Lunar Prospector archive (though not in the
present Level 1 archive) as a rough indication of the data coverage.
Other gaps may exist due to data contamination or processing
limitations.
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)
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