MPA: Meridian Photometer Array
Adapted from the VAX help files by Brian Jackel 1998/04/28
The array comprises four meridian scanning photometers, arranged in a "sideways" T, a N - S line of three stations and one to the West of the central station (originally a complete line). The Eastern line meridian instruments provides complete overlapping coverage over the full range of latitudes from approximately 55 to 73° EDFL. This includes the whole range of latitudes where particle precipitation normally occurs, from the southernmost edge of the proton oval to the dayside cleft region in the north. The western extension permits viewing the auroral oval at a location separated in local time by 1-2 hours.
These instruments operate continuously during dark hours, and provide, in quasi real time, the absolute intensities of four selected emissions. This allows one
- to monitor particle input energy and characteristics,
- to provide a cross section of the auroral oval (and for selected periods, particularly during campaigns, much higher resolution then can be obtained in the campaign mode),
- to permit comparison with spacecraft imagers, and
- to provide a measure of total auroral energy input
The instrument which makes up the Meridian Photometer Array is a meridian scanning eight-channel filterwheel photometer. Five of the eight channels measure auroral emissions (4709A, two at 4861A, 5577A, and 6300A). The three remaining channels measure background intensities (4800A, 4935A and 6250A) to correct for contamination caused by blended auroral emissions, and scattered light of solar and/or lunar origin.
The instrument scans the meridian twice each minute, stepping the mirror at 0.225 degrees per step every time the filterwheel completes a revolution. The filterwheel rotates at 120 r.p.m. This produces 510 samples per scan per channel. Each sample is corrected for non-linearity (pulse pile-up) and dark count. Each scan is binned into seventeen latitude bins and the two 4861A channels and the two corresponding background channels (4800A and 4935A) are averaged. Finally, the resulting scan starting at the even minute and the scan starting 30 seconds later are averaged. For 4709A, 4851A and 5577A, bin boundaries are computed assuming an altitude of 110 km and the resulting bins are 0.5 degree latitude wide. For 6300A, the emission altitude is assumed to be 250 km and the resulting bins are 1.0 degreee latitude wide. With a field of view of 4 degrees and an entrance pupil of 10 cm, the sensitivity is about 25 counts per Rayleigh for each averaged measurement. Dark count is measured once an hour on the half hour and a calibration is performed once an hour on the hour using an internal calibration source.
The instruments operate during the period when the solar zenith angle is greater than 96 degrees. Operation is fully automatic, using a built-in solar ephemeris routine and a two-level dawn-dusk sensor for controlling the operating periods. The interference filters and the photomultiplier are temperature controlled. Backup heaters are provided to protect the interference filters in case of prolonged power outages. Every two minutes, each instrument transmits two scans in the four auroral emissions and two background channels and the housekeeping data. The housekeeping data include the instrument dark count, the auto-calibration data and other instrument status information, such as temperatures and voltages.
A separate campaign port (RS-232, 4800 baud) provides a readout of all 510 samples for each channel as well as the housekeeping data.