The upper atmosphere and 'plasmasphere' of the Earth are a mixture of ionized plasmas, and ordinary 'neutral' atoms with their normal compliment of electrons intact. Space physicists are very interested in the ionized atoms because they are powerful tracers of the dynamics of the magnetic fields and plasmas that exist in the space environment. Without a complete knowledge of where the ions are located, and therefore the plasmas, it is impossible to construct reliable models of how the complex space environment near the Earth changes from moment to moment during solar storms and other geomagnetic disturbances.
The Neutral Atom Imagers do not 'image' these plasmas directly, but instead detect the neutral atoms that the ions collide with as they move along the magnetic field of the Earth. The entrance apertures of each of the three instruments, HENA, MENA and LENA are 8 degrees x 8 degrees. At any given instant, the windows are pointed in a known direction in space. The windows allow the neutral atoms to penetrate deep into the three instruments where they are converted into negatively-charged ions, and then bent around by a magnetic field to impact a detector. Depending on the mass of the atom, the ion will be bend around by different amounts so that, from their impacts upon the detector, one can identify their masses and therefore what element they are.
The actual data from these three instruments paints a picture of the sky in three neutral atom energy intervals, showing where these atoms are coming from. From this data, and the known shape of the Earth's magnetic field at the time the images are made, it is possible to recover with mathematics, where the ions were located that originally collided with the neutral atoms and sent them on their way into the three instruments.
From this recovered map of the ions, space scientists can follow how these particles flow through the magnetosphere from one part of the system to another during various types of geomagnetic and solar storm events.
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