| DESCRIPTION |
Instrument Overview
===================
This is the ASPERA-3 (Analyzer of Space Plasmas and Energetic
Atoms - 3rd version) instrument description.
Analyzer of Space Plasmas and Energetic Atoms, 3rd version
----------------------------------------------------------
(ASPERA-3)
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Abstract. The general scientific objective of the ASPERA-3
experiment is to study the solar wind-atmosphere interaction and
characterize the plasma and neutral gas environment in the near-
Mars space through energetic neutral atom (ENA) imaging and
local charged particle measurements. The studies to be performed
address the fundamental question: How strongly do the
interplanetary plasma and electromagnetic fields affect the
Martian atmosphere? This question is directly related to the
problem of Martian dehydration. The ASPERA-3 instrument
comprises four 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-3 main unit. IMA provides ion measurements
in the energy range 0.01 - 40 keV/q for the main ion components
H+, H2+, He+, O+, with 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 5
deg x 11 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 - 20 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-3 experiment. IMA provides ion measurements
in the energy range 0.01 - 40 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 4.6 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-3 experiment fulfills the Mars 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
Martian 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-Mars 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-3 experiment are:
1) Scientific objective: Determine the instantaneous global
distributions of plasma and neutral gas near Mars
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 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 ion bombardment
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)
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: Search for the solar wind-Phobos
interactions
Associated measurements: ENAs originating from Phobos
Measurement requirements: ENA measurements in the energy range
tens eV - few keV with 4pi coverage. ENA flux
> 10**4/(cm**2-s-keV)
6) 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-3 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.
All sensor units were fully calibrated, and some preliminary
functional tests were made in the fully mounted, flight,
configuration in June 2002. The final functional tests were
successfully carried out during the retrieval period 18 November
- 9 December 2002.
The Neutral Particle Imager, NPI, was calibrated in Nov-Dec 2001
at the IRF ion source in Kiruna. The sensitivity of the
instrument and the characterization of the acceptance field of
view were obtained using ions (e.g. N+, H2O+, and H+). The ion
deflector properties versus energy were calibrated for various
deflector voltages. The integral efficiency of the secondary
surface were found to range between 2% and 24% (MCP bias
dependent). Azimuthal field-of-view slightly broader than nominal
(13.5 deg). NPI-calibration performance as expected. The ideal
NPI field-of-view, 4pi, is covered in half a scan of the scanning
platform.
The Neutral Particle Detector, NPD, is a completely new design
using secondary emitting surfaces and a geometry that has not
been flown before. NPD therefore underwent extensive
characterisation tests and calibrations during the spring of
2002. The results of the calibrations at the IRF calibration
facility in Kiruna were very successful, the NPD performance
surpassing expectations. NPD is even more sensitive than
expected. 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 MSSL in
London, fall 2001. The energy resolution was found to be better
than expected, which is an advantage for studying the narrow
electron peaks expected as a result of the solar impact on the
ionosphere and upper atmosphere. The geometric factor is a factor
of five less than nominal, but the loss of sensitivity is, for
this mission, considered to be well compensated by the improved
energy resolution of ELS. Calibrations and tests are considered
to be successful.
IMA was successfully calibrated January to March, 2002.
Performance largely as expected, except that the upper energy
limit was lowered from 40 keV to 30 keV. Mass resolution, an
energy and angular characteristics also as expected.
Operation of ASPERA-3
=====================
The ASPERA-3 experiment contains four sensor units and the
scanner. Each sensor unit measures different components of the
near-Mars plasma and can be operated in different modes. To
handle available power and telemetry resource requirements in the
most efficient way and to inhibit too large number of individual
modes, there are eight basic TM modes (macro modes): (1) OFF
mode, (2) Safe mode, (3) Housekeeping mode, (4) Calibration mode,
(5) Low mode, (6) Normal mode, (7) High mode, and (8) Burst mode.
(1) OFF mode. The instrument is off although the external heaters
are on and controlled by the instrument thermistors.
(2) Safe mode. At experiment power switch on, the instrument
enters to a safe mode. In the safe mode the software is run
in PROM although the software allows command execution,
housekeeping TM generation, RAM dumping and jumping to the
RAM code. The instrument also enters to the safe mode in the
following cases:
* the checksum of the RAM code fails
* watch dog is not reset
The safe mode is a fully operational mode, and the instrument
is listening for other commands.
(3) Housekeeping mode. In this mode none of the ASPERA-3 sensors
are taking scientific data and the DPU delivers housekeeping
data to OBDH. This mode is to monitor the instrument status.
(4) Calibration mode. In this mode each of the different sensors
is switched on individually for check-out and in-flight
calibration purposes.
(5-8) Low, Normal, High, Burst modes. These modes are for the
scientific data taking. 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-3 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
log-compression of 16-bit words to 8-bit words, masking, and
look-up tables (NPD). The processed and formatted data are loss-
less compressed by the USES algorithm (Universal Source Encoding
for Space, CCSDS 111.0-W-2).
Principal Investigator
======================
PI: Prof. Rickard Lundin
Co-PI: Dr. Stas Barabash
Both at Swedish Institute of Space Physics (IRF), Kiruna, Sweden
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