The Magnetospheric Plasma Analyzer (MPA) consists
of a single electrostatic analyzer (ESA) coupled to an array of channel
electron multipliers, and it measures three-dimensional energy/charge
distributions of both ions and electrons in a range of ~1 eV/q to
>40 KeV/q. For use at synchronous orbit, the instrument was designed
for high sensitivity with moderate energy and angular resolutions. The
ESA is composed of a set of spherical plates such that the bending
angle from the center of the entrance aperture is a constant 60 degrees,
independent of the polar angle of entry. Electrons and ions are analyzed
alternately with this single ESA. After leaving the ESA, the particles
are accelerated into an array of six spiral-configuration channel
electron multipliers (CEMs). Each CEM covers a separate polar angle
field of view (FOV). The FOVs are fan-shaped and centered at +/- 11.5,
34.5, and 57.5 degrees polar angle, where 90 degrees is the spin axis
direction (actively controlled to point to the center of the earth).
Three-dimensional measurements are obtained by using spacecraft rotation,
at a nominal 6 rpm, to sweep the FOV through the full 360 degree range of
spacecraft azimuth angle while recording counts in the six polar angle
FOVs. Measurements are made in standard cycles at each of 24 angles (15
degree spacing) in azimuth during each 10 s spacecraft revolution.
Approximately 92% of the unit sphere is observed by the MPA. Normal
instrument operation is composed of five types of cycles arranged in an
eight-cycle sequence that repeats every 86 s. This provides both two-
dimensional and three-dimensional distributions, with varying resolutions.
For more details of the instrument, see the paper by S. J. Bame et al.,
Rev. Sci. Instrum., 64, (4), pp. 1026-1033, 1993.