Sheet / PSBL
Region: Polar Cap
Caution level: High
Caution must be used when using EFW data from the polar cap. Cold
ion temperatures and higher flow velocities often create a strong electrostatic
wake around the spacecraft. The electric field measured is then the
sheath electric field, and not the geophysical one. Such data must therefore
not be used for scientific purposes (other than perhaps to study the spacecraft
sheath). Data from this region should be assumed questionable; comparison
with other instruments is always required.
|Frequency of Occurence
|Ease of detection
|Data Loss Severity
Examples of good EFW data in the polar cap
data, from 2002-02-13 (C3), shows good agreement between
both probe pairs of EFW (red, black) and EDI (green). Low densities
mean that CIS cannot get a good estimate of the flow velocity (scattered
blue dots). Agreement seems to be slightly better in the y direction,
which is not unusual. Offsets of 1.2 (P12) and 1.7 (P34) mV/m have
been subtracted in the X direction. Individual waveforms are
rather sinusiodal. More information on figure quantities is given here.
Problem: Electrostatic Wakes.
|Examples of the problem
|2002-04-28 2220 C1
Polar cap data from shortly after perigee, showing electrostatic wake structures.
The structures align with the projected magnetic field direction. Description of the plotted quantities.
2002-02-13 0130 C3
This data, from the polar cap just before the satellite encountered the auroral
zone, shows highly non-sinusoidal spins. The wake-like structures align
neither with the projected magnetic field direction nor with the projected
velocity vector. Description of the plotted quantities.
2002-03-20 2040 C3
Here is a less extreme example. Description of the plotted quantities.
|Description of the problem
|The data shows non-linear wake-like structures, and is in disagreement with simultaneous CIS or EDI measurements.
|How can it be detected?
data, when plotted either as a function of spin angle or as a function of
time, show highly non-sinusoidal behaviour. The wake usually shows
up as a pair of spikes that usually (but not always) occur near the field
aligned direction. Highly non-sinusoidal data is always suspect.
Less extreme examples may be difficult to see. Comparison with
other instruments is always warranted.
|How can it be corrected?
perturbations are sometimes remediable using a robust spinfit technique.
Large perturbations probably cannot be corrected.
|Why does it occur?
plasma in the polar cap is cold, supersonic and of low density. When
this plasma encounters the spacecraft, a wake is left in the flow direction
(nearly field-aligned). The effective size of the spacecraft is given
by the electrostatic potential surrounding it, and not by the physical size.
Since the wire booms are also at spacecraft potential, they therefore cast
a wake of their own. The wake is therefore large, and may encompass our probes.
When this happens, a spurious wake field is measured. Some predictions
of this model include:
More information can be found in the document Cluster EFW Electric Field Measurements in the Polar Wind by Anders Eriksson et al.
- The wake is larger for larger spacecraft potential.
- The wake is worse when the plasma flow direction (~magnetic field direction) is near the spin plane,
- The effect is expected in the regime k T_i < 0.5m_i
- ASPOC should reduce this effect.
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