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Instrument: PES : Photoelectron Spectrometer
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Description
The Photoelectron Spectrometer experiment was designed to
provide information on the intensity, angular distribution,
energy spectrum, and net flow along field lines, of electrons in
the thermosphere with energies between 1 and 500 eV. The
instrument consisted of two identical oppositely directed
hemispherical electrostatic analyzers, and contained 30
operating modes. Each spectrometer had a relative energy
resolution of plus or minus 2.5% and a geometric factor on the
order of 0.001 sq cm-sr, independent of electron energy. Three
separate energy ranges could be measured: 0 to 25, 0 to 100, and
0 to 500 eV. Measurements from these intervals could be
sequenced in five different ways. Data could be taken from
either sensor separately, or alternately with time resolution
varying from 0.25 to 8 s. There were two deflection voltage
scan rates determined by the spacecraft clock. This voltage was
changed in 64 steps, and was done at 4 or 16 steps per telemetry
frame. With 16 frames/s, this allowed a choice of either one
64-point spectrum, or four 16-point spectra in one second. The
longest (8 s) cycle of data involved observations using
increasing voltage steps for the lowest, middle, lowest, then
highest energy ranges (in that order) for 1 s each. A repeat
for decreasing voltage steps completed the cycle.

[Summary provided by NASA]