The electron sensor measures environmental electrons in the temperature range of 500 to 8000 deg K and in the density range from 10^6 to 10^3 (s/c in sunlight) or 10^2 (s/c in darkness). It makes one complete measurement cycle each 8 sec consisting of 4 sec of increasing bias voltages and 4 sec of decreasing voltages. Each 4 sec of data yield one measurement of electron temperature and density.
The electron sensor is a spherical Langmuir probe with an outer gridded sphere of diameter 2.25 inches and a solid, inner sphere of diameter 1.75 in. The sensor weighs 0.2 lbs.
The outer sphere is held at the potential of the ion sensor array plus a set of voltages from +4 V to -4V which are stepped through in 4 seconds. These voltages act as an electrostatic filter. The variation of collected current vs. applied voltage is used to determine the temperature of the ambient electrons. Once each 512 sec, the electron probe's outer grid is stepped from -6V to +28V. Under certain conditions, this cycle can interfer with the ion driftmeter data; thus routine analysis of the dm data drops 4 sec of data each 512 sec.
The inner sphere is electrostatically biased by +20V to collect all electrons which pass through the outer, gridded sphere and none of the ambient ions. In addition to the environmental electrons, the sensor collects photoelectrons from the spacecraft and photoelectrons from the gridded sphere which complicates the data analysis.
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