POLAR ELECTRIC FIELD INSTRUMENT
by Forrest Mozer
April 1, 1996
WHAT IS THE PROBLEM?
At plasma densities greater than about 800 particles/cm3, which occurs on Polar only in the plasmasphere near perigee, the preamplifiers associated with the spheres in the four wire booms on the Polar satellite oscillate. This produces a line at what is assumed to be the plasma frequency, low frequency noise, and occasional harmonics of the plasma frequency. The preamplifiers associated with the spheres in the on-axis booms do not oscillate.
These phenomena can be seen in the accompanying figure between about 1610 and 1630 UT. The bottom curve of this figure is the potential difference between sphere 1 and the spacecraft. Because of biasing, the sphere is near the local plasma potential, so the potential difference between the sphere and the spacecraft is about equal to the negative of the spacecraft floating potential. Since the spacecraft floating potential is a function of the plasma parameters, the plasma density may be estimated from this data. Near the beginning and end of the figure or in the auroral zones between about 1450 and 1550 UT, the density is about 1 particle/cm3. After 1600 UT the density increases until the oscillation occurs near 1610. The oscillation is apparently rectified by the non-linear Langmuir probe characteristic of the sphere, so the potential difference between the sphere and the spacecraft decreases by a fraction of a volt when it occurs. The top two curves are the potential differences between the two sphere pairs that are at the ends of the long wire booms. These potential differences vary at the spin frequency due to their rotation in an external DC field, but this variation cannot be seen in the figure due to the compressed time scale. During the preamplifier oscillation, there are relatively small amplitude signals in these potential differences due to the oscillation. The remaining plot is the potential difference between the two on-axis spheres, and this does not show any sign of the preamplifier oscillation.
On the particular orbit illustrated in the accompanying figure, the preamplifier oscillations occurred for 20 minutes of the 17.5 hour orbit period.
WHY DO THE PREAMPLIFIERS OSCILLATE?
The present hypothesis is that the plasma has an inductive component, L,
near the plasma frequency and there is feedback from the amplifier output to
the input through the stray capacitance in the circuit, C. When the plasma
frequency, 
p , is such that LC
p2 = 1, the amplifier circuit becomes an
oscillator. The stray capacitance and plasma inductance are such that this
occurs only at high densities.
The on-axis spheres do not oscillate, presumably because they are in a lower density plasma since they are within the spacecraft potential where the density is less, or because their stray capacitance is less due to the different mechanical configuration associated with the rigid booms.
CAN THE INSTRUMENT BE CONFIGURED TO PREVENT THIS OSCILLATION?
Diagnostic experiments on the sphere pair that is not being used by the plasma wave experiment continue to be performed in an attempt to find values for the bias current and the guard and stub potentials that can control this oscillation. From the present data, it does not appear likely that this can be done.
CAN THESE OSCILLATIONS HARM THE EFI HARDWARE?
From the accompanying figure the answer is obviously NO. The general auroral zone field is larger than that produced by the oscillations and the electrostatic shocks observed near 1512, 1530, and 1548 UT produce signals that are at least an order-of-magnitude larger than those produced by the oscillation.
DO THESE OSCILLATIONS IMPACT EFI SCIENCE?
There is no negative impact on the EFI science because the oscillations occur in uninteresting regions where the density is high. In interesting regions, such as the auroral zones near perigee in the accompanying figure, the density is decreased below the critical value required for oscillation by the physics that is occurring.
There is an important positive impact on the EFI science because the oscillations may be used to understand the coupling of Langmuir probes in space plasmas. For example, the value of the inductance required to cause oscillations at the observed frequencies will be determined in the laboratory by putting inductors across flight spare components. In this way, the hypothesis for the oscillations can be tested and the inductive component of the plasma coupling can be measured. These results are significant to Langmuir probe theory and, to the best of our knowledge, this coupling has not been measured in space conditions.
CAN THESE OSCILLATIONS OCCUR ON CLUSTER?
Since Cluster has essentially the same preamplifiers and mechanical attachments as does Polar, it is important to determine whether these preamplifier oscillations can occur on Cluster. The Cluster perigee of 4 RE is reached in the accompanying figure at 1415 and 1730 UT. At these times the density is less than 1 particle/cm3, so it is highly unlikely that the Cluster electric field experiment will oscillate.
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