The MPA instruments were designed and built to measure the three-dimensional plasma electron and ion distributions at geosynchronous orbit [Bame et al., Rev. Sci. Instrum., 1993]. MPAs are fielded by Los Alamos National Laboratory, in collaboration with Sandia National Laboratory, on a series of geosynchronous spacecraft. To date, MPAs have been lauched on spacecraft 1989-046, 1990-095, 1991-080, and 1994-084; the number before the dash in these "international designators" gives the year of launch. While there are no magnetic field measurements made on these spacecraft, the field orientation is frequently derivable from the shape of the MPA particle distributions [Thomsen et al, JGR, 1996].
The MPA is a spherical-sector electrostatic analyzer with a bending angle of 60°. The field of view is divided into six separate but contiguous detectors covering the range of polar angle from about 25° to 155°, and the satellite spin allows the instrument to view 360 degrees in azimuth, which is divided into 24 sectors of 15 degrees. Thus in one spin the MPA views ~92% of the unit sphere, divided into six polar by 24 azimuthal view directions. The spin axis of the spacecraft points continuously at the center of the Earth, so the two polar angle detectors which view nearly perpendicular to the spin axis give very complete pitch angle coverage. While the spacecraft spins through a 15° azimuthal sector, the MPA plate voltage is swept through 40 logarithmically spaced energy channels ranging from ~40 keV/e down to ~1 eV/e . A complete three-dimensional (40 energies x 24 azimuths x 6 polar angles) distribution is obtained in one 10-s spin. Since the same analyzer is used for both ion and electron measurements (by changing the polarity of the plate voltage and channel electron multiplier bias [Bame et al., 1993]), the ion and electron distributions are measured alternately. In 86 s, the instrument cycles through one three-dimensional electron distribution and two three-dimensional ion distributions, as described above, as well as three two-dimensional electron distributions and two high-angular-resolution modes [Bame et al., 1993].
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