Instrument Overview
     ===================
     This is the ASPERA-4 (Analyzer of Space Plasmas and Energetic Atoms -
     4th version) Electron Spectrometer (ELS) instrument description.
 
     Analyzer of Space Plasmas and Energetic Atoms, 4th version (ASPERA-4)
     Electron Spectrometer (ELS)
     ---------------------------------------------------------------------
     Abstract. The general scientific objective of the ASPERA-4 experiment
     is to study the solar wind-atmosphere interaction and characterize
     the plasma and neutral gas environment in the near-Venus space
     through energetic neutral atom (ENA) imaging and local charged
     particle measurements, in unprecedented detail and accuracy. The
     studies to be performed would lead to a better understanding of the
     origin and evolution of the planet, and particularly of induced
     escape mechanisms responsible, among others, to the Venusian
     atmosphere dehydration process.
 
     The ASPERA-4 experiment is an instrument comprised of four different
     sensors; two ENA sensors, electron and ion spectrometers. This
     description only includes the electron spectrometer (ELS) instrument.
     The electron spectrometer (ELS) is a standard top-hat electrostatic
     analyzer in a very compact design. The ELS is located on a scanning
     platform providing a 4pi coverage (maximum possible).
 
     Electron Spectrometer (ELS)
     ---------------------------
     The ELectron Spectrometer (ELS) provides electron measurements in
     the energy range 0.01 - 20 keV. The intrinsic field of view is
     4 deg x 360 deg. The 360 deg aperture is divided into 16 sectors.
     The sensor is a standard top-hat electrostatic analyzer in a very
     compact design. The Venus Express Electron Spectrometer was built
     at Southwest Research Institute as the flight spare for the Mars
     Express mission under a NASA contract. IRF obtained approval from
     NASA to use the Mars Express ELS flight spare for the Venus Express
     ASPERA-4 ELS flight model.
 
     Scientific Objectives
     =====================
     The ASPERA-4 ELS experiment helps fulfill the Venus Express mission
     objective of studying the interaction of the atmosphere with the
     interplanetary medium by in situ measurements of electrons in order

        (a) to complement the ENA images (electrons cannot be imaged)
        (b) to study local characteristics of plasma (dynamics and fine
            structure of boundaries), and
        (c) to provide undisturbed solar wind parameters.

     The main scientific objective of the ASPERA-4 ELS instrument is to
     define the local characteristics of the main plasma regions.
     Associated measurements: Electron measurements of hot plasma
     Measurement requirements: Electron measurements in the energy range
     few eV - tens keV with 4pi coverage.


     Calibration
     ===========
     Calibration of the ASPERA-4 sensors can be divided up in:

     1. Characterization, tests and selection of detectors (MCPs and
        secondary emitting surfaces).
     2. Characterization and final calibration of the integrated sensor
        units.
     3. Functional tests of the sensors in the fully mounted (flight)
        configuration.

     The Electron Spectrometer unit, ELS, was calibrated at Mullard Space
     Science Laboratory (MSSL), University College in London. The setup
     for the calibration is identical to the one that was used for the
     calibration of the Mars Express instrument. The calibration facility
     provides a wide area photoelectron beam at energies ranging from a
     few eV to 15 keV with variable beam intensities from a few Hz to
     several MHz. Tests were carried out to study the following parameters
     of the instrument: Analyser (Concentricity, K-factor, Angular
     acceptance and Energy Resolution), Detector (Operational regime,
     Rate response, Gain uniformity/QE) and UV response. First results
     have shown significant differences in instrumental properties as a
     function of the anode sector position due to a misalignment effect
     in the fabrication process. Considering the mechanical imperfections
     of the instrument, the laboratory calibrations were critical for
     defining the instrument response.

     Operation of ASPERA-4
     =====================
     The ASPERA-4 experiment contains four sensor units and the scanner.
     Each sensor unit measures different components of the near-Venus
     plasma and can be operated in different modes. The modes differ from
     each other in the total amount of data produced and the structure of
     TM packages although individual settings defining the sensor
     configurations might be the same for different modes. The choice of
     the instrument operational mode for each phase of the mission is due
     to available power and telemetry as well as scientific requirements.

     The scanning platform has three operational modes: scanning mode,
     stepping mode, and fixed position mode. In the scanning mode, the
     platform performs scans with three pre-selected speeds 32, 64, and
     128 sec in one 0 deg - 180 deg scan. In the stepping mode the
     platform moves in steps through the angle defined by a command. The
     time the platform rests in each position is also commanded. In the
     fixed position mode the platform moves to a commandable position from
     0 deg to 180 deg and rests there until the scanner mode changes.

     All four ASPERA-4 sensors, ELS, NPI, NPD1 and NPD2, IMA, can be run
     independently although the individual sensor bit rates are set by a
     macro command. The raw data are compressed by integration over time,
     energy, azimuth, mass as well as using logarithmic compression of the
     count values, RICE compression of final spectrum, masking, and
     look-up tables (NPD).

     Principal Investigator
     ======================
     PI: Dr. Stas Barabash
     at Swedish Institute of Space Physics (IRF), Kiruna, Sweden
     Co-PI: Dr. Jean-Andre Sauvaud
     at Centre d'Etude Spatiale des Rayonnements (CESR), Toulouse, France

     Co-Investigators
     ================
     Dr. David Winningham
     at Southwest Research Institute (SwRI), San Antonio, Texas
     Dr. Rudy Frahm
     at Southwest Research Institute (SwRI), San Antonio, Texas
     Professor Andrew Coates
     at Mullard Space Science Laboratory (MSSL), Surrey, United Kingdom"