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Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4)

NSSDCA ID: 2005-045A-06

Mission Name: Venus Express
Principal Investigator:Dr. Stas Barabash

Description

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) is designed to study energetic neutral atoms (ENAs), ions and electrons from Venus orbit. The scientific objectives of the experiment are to: Investigate the interaction between the solar wind and the atmosphere of Venus; characterise quantitatively the impact of plasma processes on the atmosphere; determine the global distribution of plasma and neutral gas; identify the mass composition and quantitatively characterize the flux of the out flowing atmospheric materials; investigate the plasma domains of the near Venus environment; and provide undisturbed solar wind parameters.

The ASPERA-4 consists of four instruments, the Neutral Particle Imager (NPI), the Neutral Particle Detector (NPD) the Electron Spectrometer (ELS) and the Ion Mass Analyzer (IMA) and has two components. The Main Unit holds the mechanical scanner, digital processing unit, NPI, NPD, and ELS. The IMA is mounted separately. The mechanical scanner sweeps the three sensors mounted on it through 180 degrees to give the ASPERA-4 instrument 4 pi steradian (unit sphere) coverage when the spacecraft is 3-axis stabilized. The operational rotation rates are 1.5, 3.0 and 6.0 degrees per second. The system offers an angular positioning accuracy of 0.2 degrees.

In the Neutral Particle Imager, incoming particles pass between two 150 mm diameter discs separated by 3 mm with a 5 kV potential between them. Charged particles are deflected by the electric field and captured, but neutral particles pass between the discs. The space between the discs is divided into 32 azimuth collimators with an aperture of 9 degrees by 18 degrees each. Neutrals that pass through the deflector system hit a 32-sided conical target at a grazing angle of incidence (20 degrees). The interaction between the neutral particles and the target results in production of secondary electrons and ions, and / or reflection of the primary neutrals. The particles leaving the target are detected by a Micro Channel Plate (MCP) stack with 32 anodes. The signal from the MCP gives the direction of the primary incoming neutral particle. The NPI covers 4 pi steradians in one 180 degree sweep by the mechanical scanner and produces an image of the ENA distribution in the form of an azimuth x elevation matrix. The direction vector of 32 elements is read out once every 62.5 ms.

The Neutral Particle Detector consists of two identical pinhole cameras each with a 90 degree Field of View (FoV) in the instrument azimuth plane and arranged to cover a FoV of 180 degrees. Particles approaching the pinholes pass between two quadrant deflector plates separated by 4.5 mm with an 8 kV potential between them. Charged particles with energies up to 70 keV are deflected, while neutrals proceed into the camera. The deflector plates also function as a collimator in the instrument elevation direction. The collimated ENA beam emerging from the 4.5 x 4.5 mm pinhole hits a target at a grazing angle (20 degrees) and causes secondary electron emission. The secondary electrons are detected by one of two MCP electron multiplier assemblies. The MCP output provides a start signal to measure the time of flight of the ENAs over a fixed distance. The incoming ENAs are reflected from the target nearly specularly and travel to a second target. Again, secondary electrons are produced and detected by three more MCPs, which pass a stop signal to the time of flight electronics. The time of flight between the two targets gives the velocity of the incoming particle. Which of the three 'stop' MCPs detects the incoming particle determines its azimuth direction.

The Electron Spectrometer determines the energy spectrum of incoming electrons in each of sixteen 22.5 degree sectors. It is based around a spherical section electrostatic analyser of 'top hat' design. The electrostatic analyser consists of two concentric hemispherical electrodes, the outer of which has a central hole, through which electrons are admitted, covered by the 'top hat' and collimator. Electrons arriving from any azimuth angle and within the elevation field of view of the collimator pass under the 'top hat' and are deflected through the central hole in the outer hemisphere by a positive potential on the inner hemisphere. The electrostatic field between the hemispheres will deflect electrons having an energy in a particular range such that they travel between the electrodes. Electrons with energies outside the selected range will be captured. These energy band filtered electrons exit the annular gap between the hemispheres and hit a MCP electron multiplier. Beyond the MCP, the electrons strike one of sixteen anodes, each defining a 22.5 degree sector of incident azimuth angle. By varying the electrostatic potential between the hemispheres of the electrostatic analyser, the energy of the electrons selected by the filter can be changed. The voltage applied to the inner hemisphere is swept once every four seconds and the number of anode hits per sample interval is recorded to give an energy spectrum for the incoming electrons in each sector.

The Ion Mass Analyser determines the mass spectrum of incoming ions in a selectable energy range, mass range, and spectral resolution. Ions arriving at the IMA pass through a grounded grid and enter the deflection system comprising two curved, charged plates that deflect ions arriving in the instrument elevation range of ± 45 degrees and from any azimuth angle into the entrance of the electrostatic analyser. This consists of two concentric hemispheres with a variable electric field between them. Ions that lie within the energy pass band of the analyzer travel between the hemispheres, exit the annular space separating them and travel on towards the magnetic mass analyser. The electrostatic potential between the hemispheres determines the energy range of the ions. In the magnetic mass analyser, the ions pass through a static, cylindrical magnetic field, which deflects light ions towards the center of the cylinder more than heavy ones. An electrostatic potential can be applied between the electrostatic analyser and the magnet assembly to accelerate the ions. Varying this potential allows selection of the mass range to be analysed and the mass resolution. As the ions leave the magnetic mass analyser they hit a MCP. The electrons exiting the MCP are detected by an imaging anode system. A system of 32 concentric rings measures the radial impact position, which corresponds to ion mass and 16 sector anodes measure azimuthal impact position, which corresponds to ion azimuth entrance angle.

ASPERA-4 is based on the Mars Express ASPERA-3 experiment.

Alternate Names

  • ASPERA-4
  • VenusExpress/ASPERA-4

Discipline

  • Planetary Science: Fields and Particles

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Stas BarabashPrincipal InvestigatorSwedish Institute for Space Physicsstas.barabash@irf.se
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