Overview:
  ========

    The Electron Reflectometer high resolution (ERHR) data set is a time
    ordered series of electron measurements from the Lunar Prospector
    (LP) Mission.  Each record consists of a time tag with 2 scalar data
    points representing measurements of the electron flux in 2 different
    energy channels, typically 140 and 340 eV, with an energy resolution
    of 25%. Each data point is a measure of the electron flux (cm-2 sec-1
    ster-1 eV-1) averaged over 4-pi steradians.  These data are
    accumulated over 1/2 spin (~2.5 sec) and are measured continuously.

    These data are intended to be used in conjunction with LP
    Magnetometer (MAG) data records, which provide the magnetic field
    vector as a function of time.  Electrons travel along the magnetic
    field lines in tight helices (few km radius) at high speed (roughly
    one Mars 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.


  Parameters:
  ==========

    The ERHR data are provided as differential electron flux in units of
    particles per square cm per second per steradian per electron volt.
    ERHR measurements are recorded at 2 different energies, typically 140
    and 340 eV.  The energy sweep was changed several times during the
    mission for both instrumental and scientific reasons.  A record of
    these sweep changes is provided in a detached table, with energies
    given in electron volts.


  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 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 1/2
    spin), and the second dimension is energy (2 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.  (The ER low resolution data set can
    be used for this purpose.)  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).

  Media/Format:
  ============

    Data are archived on CDROMs in level 1 compliance with the ISO
    9660 standard. Most CDROMs contain 23 days of data. The data
    set consists of two types of ASCII tables: data and energy
    bins. The data are provided as ASCII ''tables'' of 1 day
    duration. Fluxes are provided for 2 energy bins (~140 & 340 eV)
    collected over a single spacecraft spin (~5 sec). The electron
    reflectometer section of the instrument samples 4 pi steradians
    twice each spin period. The second table is a collection of the
    energy bin values. The instrument can be commanded to acquire
    data in a number of different sets of energy bins. In practice,
    the energy bins are not changed frequently. The times and new
    set of energy bins are recorded in this second table.


    ER High Resolution Time Series Data:
       naming convention: EHyymmdd.TAB

       time:             Time (UTC) in standard PDS format.
       high_res_spec:    Array[2] giving omnidirectional electron
                         flux in 2 energy bins.
       Format: time (A19), high_res_spec (1x,(2(1x,E9.3)))


    Energy Bin Change Table (E_BINS.TAB):
       time:             Time (seconds since Jan 1, 1970) of the
                         change to a new instrument configuration.
       UTC:              Time (in PDS standard time format) of the
                         change to a new instrument configuration.
       energy_bin:       Array[2] giving 2 energy bins.
       Format: time (F15.0), UTC (1X,A20), energy_bin (F9.3,1x,F9.3)


  Ancillary Data:
  ==============

    There are several ancillary data files provided with this
    archive. These include:


       Spacecraft Attitude data    LP-L-ENG-6-ATTITUDE-V1.0
       Spacecraft Ephemeris data   LP-L-6-EPHEMERIS-V1.0
       Spacecraft Position data    LP-L-6-TRAJECTORY-V1.0
       Spacecraft Command logs     LP-L-ENG-6-COMMAND-V1.0


    These data sets provide additional information about the state
    of the spacecraft and the instrument during data acquisition
    that may aid in the scientific analysis of this data set.


  Coordinate System(s):
  ====================

  Software:
  ========

    There are no software provided with this data archive.

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 ERHR 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 (roughly one Mars 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 ERHR 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.
    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.  No
    corrections are made for spacecraft potential effects.


  Data Coverage:
  =============

    ERHR data are obtained continuously; however, telemetry gaps do
    occur. A table of gaps in the raw merged telemetry data
    (OUTAGES.TAB) is provided with this data set as a rough
    indication of the data coverage. Other gaps may exist due to
    data contamination or processing limitations.