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Instrument Descriptions

The Magnetospheric Plasma Analyzer (MPA) was designed to minimize weight, power, and volume while providing comprehensive measurements of plasma conditions. Similar instruments are on board all three of the constellation of geosynchronous spacecraft 89-046A, 90-095A, and 91-080B. The 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 this 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.

The MPA KP data values are preliminary and have a number of problems which will be corrected when we revise our algorithms and re-process the data. However, that could be a while in coming. It is to be understood that the MPA data set contains very preliminary values (not publishable!) and any investigators interested in using them should contact us before embarking on any extensive study using them.

Michelle Thomsen (S:ESSDP1::thomsen)

For more information, see the MPA homepage.

The Synchronous Orbit Particle Analyzer (SOPA) consists of three nearly identical silicon solid state detector telescopes, pointed at 30, 90, and 120 degrees to the satellite's earth-centered spin axis. Similar instruments are on board all three of the constellation of geosynchronous spacecraft 89-046A, 90-095A, and 91-080B. The working end of the telescope consists of a very thin front silicon detector, D1, followed by a thick back detector, D2. The D1 sensors are mounted with the thin aluminum contact out. The thinner than usual Al contact was chosen to minimize the entrance deadlayer to allow as low a proton threshold as possible. Measurements show that the deadlayer is approximately 30 micrograms/cm**2. The detector stack is surrounded, except for the aperture, by passive low-Z (aluminum) and high-A (copper) shielding, which excludes side-penetrating protons up to about 65 MeV and electrons up to 6 MeV. The front collimator baffle is designed to require at least two-fold scattering of particles not in the acceptance angle of the detector to encounter the D1 detector. This provides an extremely sharp angular cutoff of incident particles. The full acceptance angle of the telescopes is about 11 degrees. Each telescope has a geometrical factor of 8.49 E-4 cm**2 sr for ions, and 1.09 E-3 cm**2 sr for low-energy electrons. A single rotation requires about 10 s, and in that time, 64 cuts of the unit sphere are taken by the three telescopes in a certain pattern. Passive cooling keeps the telescope temperatures within the range -15 to +5 C. In this range, essentially all of the noise associated with leakage current in the surface barrier detectors is eliminated. Even so, the high capacitance of the D1 sensors (~500 pF) requires setting the first energy threshold relatively high. The numerous thresholds and logic channels (including anti-coincidence) result in identification of differential fluxes of protons from 50 KeV to 50 MeV. Differential fluxes of electrons are determined from 50 KeV to 1.5 MeV, with integral flux above 1.5 MeV. The differential flux range for alpha particles is 0.5--1.3 MeV; for CNO, 1.5--3.55 MeV; for carbon, 5.0--13 MeV; for nitrogen, 6.0--14 MeV; and for oxygen, 7.2--15 MeV. Integral ion fluxes are determined for sulphur above 8 MeV, and for bromine above 15 MeV. A sun sensor with three collimators that overlap the fields-of-view of the three telescopes provides input to tag the pulse pairs that are potentially contaminated by sun-generated pulses from D1. For more details of the instrument, see the paper by R. D. Belian et al., J. Geophys. Res., 97, A11, pp. 16897-16906, 1992, from which this information was obtained.

For more information see the Energetic Particle Homepage.

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