Spectrometers
MIMS
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]