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4. Current BiasingSo the floating potential is a function of the density and the temperature. Well, that shouldn't be a problem, right? As long as the 2 probes are at the same potential with respect to the plasma around them (i.e. the temperature and density are the same), then we should still be able to take a differential measurement, and therefore get the electric field in the plasma.This is fine in a dense plasma. In fact, this is what has been done on lower-orbit missions such as Freja. Unfortunately, there's a catch in the less-dense plasma of the magnetosphere. That catch is that at a plasma density of 100 per cc, the IV curve looks like this: 
While at 1 per cc, it looks like this: 
That is, it is totally dominated by the photoelectrons. Instead of floating at 3 V, the probes float at 17 V. Worse than that, the floating potential varies enormously with small fluctuations in the currents. This is clearly not a good situation for measuring electric fields. So, what do we do? We draw a bias current.
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The
EFW instrument draws a current, called the bias current, from the probes.
The total of the plasma and photoelectron currents must then equal
the bias current (instead of zero), so the probe potential adjusts to the
operating point shown to the right (assuming a bias current of 140 nA).