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The auroral photometer (AP) is a two-channel broad-band instrument that is used to determine the energy deposited in the upper atmosphere by energetic auroral electrons. It is similar to airglow photometers developed by LASP and flown on OGO-5 and -6 in the late 1960's. The channels consist of two Hamamatsu phototube detectors, a UV filter for each channel, and a field of view limiter for each channel. Both channels have circular fields of view, 11 degree full-cone. The detectors are identical phototubes with magnesium fluoride (MgF2) windows and cesium iodide (CsI) photocathodes. Channel A has a calcium fluoride (CaF2) filter placed in front of the detector and channel B has a barium fluoride (BaF2) filter. The combination of the CsI photocathode and the CaF2 filter produces a bandpass from 125 to 180 nm for channel A, allowing a combined measurement of the LBH bands, the OI doublet at 135.6 nm, and the OI triplet at 130.4 nm. Channel B has a 135 to 180 nm bandpass, providing a measurement of the LBH bands and the OI doublet at 135.6 nm with the exclusion of the OI triplet at 130.4 nm. The sensitivity of channel A at 130.4 nm is 23 counts/second/Rayleigh and the sensitivity of channel B at 135.6 nm is 26 counts/second/Rayleigh. The AP and UVS photomultiplier electronics are identical, resulting in significant economies in fabrication and operation.

As with the UVS, the AP is mounted with its optical axis perpendicular to the S/C spin axis. The AP produces continuous data but at a much lower rate than the UVS. Only the downward- looking 60° of each spin will be stored. The integration time for each channel is set to 63 milliseconds which provides 32 samples per channel per spin. Data from the AP are stored in its buffer, which is emptied once per spin by the S/C microprocessor.

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