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) --------------------------------------------------------------------- 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"