Written by Thomas Moreau
19/04/2007
Instrument Overview
===================
This is the ASPERA-4 (Analyzer of Space Plasmas and Energetic Atoms -
4th version) instrument description.
Analyzer of Space Plasmas and Energetic Atoms, 4th version (ASPERA-4)
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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. The Neutral
Particle Imager (NPI) provides measurements of the integral ENA flux
(0.1 -60 keV) with no mass and energy resolution but high angular
resolution. The Neutral Particle Detector (NPD) provides measurements
of the ENA flux, resolving velocity (0.1 -10 keV) and mass (H and O)
with a coarse angular resolution. The electron spectrometer (ELS) is
a standard top-hat electrostatic analyzer in a very compact design.
These three sensors are located on a scanning platform providing a
4pi coverage (maximum possible). The instrument also contains an ion
mass composition sensor, IMA (Ion Mass Analyzer). Mechanically, IMA
is a separate unit connected by a cable to the ASPERA-4 main unit.
IMA provides ion measurements in the energy range 0.01 - 36 keV/q for
the main ion components H+, H2+, He+, O+, and the group of molecular
ions 20-80 amu/q.
Neutral Particle Imager (NPI)
-----------------------------
The Neutral Particle Imager (NPI) provides measurements of the
integral ENA flux with no mass and energy resolution but with
4.6 deg x 11.5 deg angular resolution. The intrinsic field of view is
9 deg x 344 deg.
The sensor utilizes a graphite surface to suppress the UV background.
ENAs incident on the surface at a grazing angle of 20 deg are
reflected and/or cause ion sputtering. An MCP stack detects the
reflected particles and sputtered fragments with a discrete anode.
Neutral Particle Detector (NPD)
-------------------------------
The Neutral Particle Detector (NPD) provides measurements of the ENA
differential flux over the energy range 100 eV - 10 keV resolving
H and O with a coarse 5 deg x 30 deg angular resolution. The sensor
consists of two identical detectors each with a 9 deg x 90 deg
intrinsic field of view. The measurement technique is based on a
principle similar to NPI. ENAs incident on a surface at a grazing
angle of 15 deg are reflected and cause secondary electron emission.
The secondary electrons are transported to an MCP assembly, which
gives the START signal. The reflected ENAs hit the second surface and
again produce the secondary electrons used to generate the STOP
signal. The time-of-flight (TOF) electronics give the ENA velocity.
The pulse-height distribution analysis of the STOP signals is used to
provide a rough determination of the ENA mass.
Electron Spectrometer (ELS)
---------------------------
The ELectron Spectrometer (ELS) provides electron measurements in
the energy range 0.01 - 15 keV. The intrinsic field of view is
10 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.
Ion Mass Analyzer (IMA)
-----------------------
The Ion Mass Analyzer (IMA) is a separate unit connected by a cable
to the ASPERA-4 experiment. IMA provides ion measurements in the
energy range 0.01 - 36 keV/q for the main ion components H+, H2+,
He+, O+, and for the group of molecular ions 20 < M/q < ~80. IMA
has a 4.5 deg x 360 deg field of view. Electrostatic sweeping
performs elevation (+/- 45 deg) coverage. The IMA sensor is a
spherical electrostatic analyzer followed by a circular magnetic
separating section. A large diameter MCP with a discrete anode images
the matrix azimuth x mass.
Scientific Objectives
=====================
The ASPERA-4 experiment fulfills the Venus Express mission objective
of studying the interaction of the atmosphere with the interplanetary
medium by:
* Remote measurements of energetic neutral atoms (ENA) in order to
(a) investigate the interaction between the solar wind and
Venusian atmosphere,
(b) characterize quantitatively the impact of plasma processes
on the atmospheric evolution, and
(c) obtain the global plasma and neutral gas distributions in
the near-Venus environment.
* in situ measurements of ions and electrons in order to
(a) complement the ENA images (electrons and multiply-charged
ions cannot be imaged)
(b) to study local characteristics of plasma (dynamics and fine
structure of boundaries), and
(c) provide undisturbed solar wind parameters necessary for
interpretation of ENA images.
The scientific objectives of the ASPERA-4 experiment are:
1) Scientific objective: Determine the instantaneous global
distributions of plasma and neutral gas near Venus
Associated measurements: ENAs originating from the shocked solar wind
Measurement requirements: Measure the ENA flux in the energy range
tens eV - few keV with 4pi coverage. ENA flux > 10**4/(cm**2-s-keV)
Measure the upstream solar wind parameters
2) Scientific objective: Study the plasma induced atmospheric escape
Associated measurements: ENAs originating from the inside of the
magnetosphere
Measurement requirements: Mass resolving (H / O) ENA measurements
in the energy range up to tens keV. ENA flux > 10**3/(cm**2-s-keV)
3) Scientific objective: Investigate the modification of the
atmosphere through particle precipitation
Associated measurements: ENA albedo
Measurement requirements: Mass resolving (H / O) ENA measurements
in the energy range down to tens eV from nadir direction.
ENA flux > 10**6/(cm**2-s-keV) at 100 eV
4) Scientific objective: Investigate the energy deposition from the
solar wind to the ionosphere
Associated measurements: Precipitating ENAs
Measurement requirements: ENA measurements in the energy range
tens eV - few keV. ENA flux > 10**4/(cm**2-s-keV)
5) Scientific objective: Define the local characteristics of the
main plasma regions
Associated measurements: Ions and electron measurements of hot plasma
Measurement requirements: Ion and 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 ASPERA-4 Neutral Particle Imager, NPI, on Venus Express is a
spare model of the ASPERA-3 NPI flying on board the Mars Express
spacecraft. The sensor was calibrated at the IRF ion source in Kiruna
to characterize its response: MCP-saturation bias, dark count level,
angular response in elevation and azimuth, and efficiency. The
calibration was performed using an ion beam as a particle source
(e.g. H2O+ and H+). One sector (number 4) was thoroughly
investigated, and a scan with the calibration beam in the central
plane of all sectors was used to find the relative sensitivity of the
other 31 sectors, which varies significantly. NPI-calibration
performance is as expected. The ideal NPI field-of-view, 4pi, is
covered in half a scan of the scanning platform. The NPI sensor was
also calibrated against Lyman-alpha photons (121.6 nm) at the
University of Arizona. The measured UV suppression efficiency is of
3x10**-5, which is somewhat higher than what was previously expected.
The Neutral Particle Detector, NPD, developed for the Venus Express
mission ASPERA-4 experiment underwent extensive characterisation
tests and calibrations in 2004. The NPD calibrations included
determining the MCP characterization to obtain the nominal bias,
efficiency, angular response, geometrical factor, TOF distributions
and Pulse Height (PH) distributions analysis, energy and mass
resolution, heater test and temperature sensor response, and noise
of the NPD sensors integrated with ASPERA-4 instrument. The results
of the calibrations at the IRF calibration facility in Kiruna fully
correspond to the specified performance. The mass and energy
resolving capability of the instrument were as expected in the energy
range ~1 - 10 keV. The NPD field-of-view, 2pi, is covered after a
scan of the scanning platform.
The Electron Spectrometer unit, ELS, was calibrated at Mullar 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.
The ASPERA-4 Ion Mass Analyzer, IMA, is a flight spare of the IMA
sensor from the Mars Express mission. The ASPERA-4 IMA sensor
head design has been however slightly modified by Centre d'Etude
Spatiale des Rayonnements (CESR)/Centre National de Recherche
Scientifique (CNRS) in order to provide a better mass discrimination.
IMA was successfully calibrated at CESR in Toulouse. Performance
largely as expected. Mass resolution, energy and angular
characteristics also as expected. Calibrations and tests are
considered to be successful.
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"