The Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) for the Venus Express mission

The general scientific objective of the ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms) experiment is to study the solar wind–atmosphere interaction and characterise the plasma and neutral gas environment in the near-Venus 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 Venusian atmosphere? The ASPERA-4 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 relatively 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 analyser in a very compact design. These three sensors are located on a scanning platform providing a 4 π coverage. The instrument also contains an ion mass composition sensor, IMA (ion mass analyser). Mechanically, IMA is a separate unit electrically connected with the ASPERA-4 main unit. IMA provides ion measurements in the energy range 0.01–36 keV/ q for the main ion components H + , He ++ , He + , and the ion group with M / q 20 – 80 amu / q .