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Instrument: MIMS : Magnetic Ion-Mass Spectrometer
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Description
The magnetic ion mass spectrometer (MIMS) measures the abundances
of the ambient positive ions along the orbit of the AE satellite. It scans
the mass range from 1 to 90 amu, or portions of that range, depending on
its operational mode. The instrument consists of an entrance aperture, a
7 cm screen, flush mounted to the surface of the spacecraft, a set of
collimating slits, a magnetic analyzer and a triple detector system, each
detector consisting of a collector slit, an electron multiplier and a log
amplifier. Positive ions are rammed through the entrance grid due to the
orbital motion of the spacecraft and accelerated through the collimator
forming a beam which enters the magnetic analyzer. This analyzer consists of a
permanent magnet with a field strength of 3600 gauss in the gap. Three
ion beam trajectories are allowed through the magnet. These split the ion
beam into 3 parts in the mass ratio 1:4:16. Collector slits are placed at
the exit of the magnetic lens system corresponding to the image point of
the allowed ion trajectories. As the ion accelerating voltage is varied,
the ion beams are scanned across the collector slits forming a series of
peaks in the 103 current detector. The mass ranges covered are 1 to 4, 4 to
16 and either 16 to 90 (AE-C) or 14 to 72 (AE-D) amu at the low, mid and
high mass collectors respectively. The scan times are either 4 sec (AE-C)
or 2 sec (AE-D) for the analog short mode, or 8 sec for the analog long mode.
When the analog short mode is used, the mass ranges of each channel are cut
in half. The analog short mode is used when the satellite is spinning; the
analog long for despun operation.

The overall accuracy is better than 20%, except in very disturbed
regions of the ionosphere, or at extremely low densities. Sensitivity of
the instrument is somewhat mass dependent, varying from 1.0 ion cm-3 for
H,to ~10 ions cm-3 for O+ .

Additional information available at
http://nssdcftp.gsfc.nasa.gov/spacecraft_data/ae/aedoc/mims.txt

[Summary provided by NASA]