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- The Electron Drift Instrument
measures both the electric and
magnetic fields by tracking the path of
electron beams through space. EDI
sends a beam of electrons out into
space using each of its two Gun
Detector Units. In the presence of
magnetic fields, electrons travel in
orbits that are nearly circles, so over
the course of about half a mile, the
electron beam curves around on
itself until it comes back in to the
second Gun Detector Unit. By
measuring how long it takes the
electrons to circle back, one can
calculate the strength of the magnetic
fields through which the beam traveled.
When electric fields are present as
well, the electron beam will not make a
perfect circle, but will drift in a
predictable way as it returns. By
measuring the size of that sideways
drift, one can calculate the strength of
the electric fields.
This technique of correctly capturing
the electron beam was successfully
demonstrated by the joint European
Space Agency/NASA Cluster mission.
On MMS, the EDI will take faster
measurements than on Cluster. Its
strength, however, is not in its speed
but in its precision. Knowing the
displacement of the particles in space
due to an electron field is crucial for
accurate measurements by other
instruments aboard MMS.
If needed, EDI can also be used solely
as a detector, measuring all incoming
electrons from space as opposed to
just tracking its own specialized
electron beam. In this case, EDI can
make observations at rates of up to
1,000 times a second.
The EDI electron gun was developed
at the Space Research Institute. EDI
optics were developed at the University of Iowa.
The sensitive detector, the controlling
electronics, and the overall integration
and operation of the EDI instrument is
the responsibility of the University of
New Hampshire in Durham.
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