Earth Planets Space, Vol. 50 (No. 3), pp. 199-205, 1998
O. Norberg1, M. Yamauchi1, R. Lundin1, S. Olsen1, H. Borg1, S. Barabash1, M. Hirahara2, T. Mukai3, and H. Hayakawa3
1Swedish Institute of Space Physics, Box 812, SE-981 28 Kiruna, Sweden
2Rikkyo University, Tokyo, Japan
3The Institute of Space and Astronautical Science (ISAS), Japan
(Received July 28, 1997; Revised December 2, 1997; Accepted January 5, 1998)
Abstract: The Ion Mass Imager (IMI) is a light-weight ion mass composition instrument for the Japanese Planet-B mission to be launched to Mars in 1998. The objective of the Planet-B mission is to study the Martian environment with emphasis on the upper atmosphere interaction with the solar wind. IMI measures positive ions with energies between 10 eV/q and 35 keV/q and with a mass resolution high enough to resolve the most important ion species (H+, He++, He+, O++, O+, O2+). The instrument has a 360° field-of-view aperture and uses the spacecraft spin to cover almost the full unit sphere to obtain three-dimensional distribution functions every 4 s (half a spacecraft spin period). Particles are energy-filtered by a spherical electrostatic analyzer, and then mass-analysed by the magnetic separation method. The ions hit a microchannel plate assembly with a position sensitive anode divided into 32 mass channels. Together with 16 angular sectors, this system "images" the direction of motion and mass of ions. A pre-acceleration voltage of 0-4000 V is used to select the mass range, e.g., modes optimized for light ions (up to O+) and heavy ions (O+ to charged dust grains). A loss-less data compression algorithm is used in the in-flight processing software to optimize the amount of data that can be returned from Mars.