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.

Known problems:

Problem Type	Frequency of Occurence	Ease of detection	Remediable?	Data Loss Severity
Electrostatic wake	Frequent	Moderate	No	Complete

Examples of good EFW data in the polar cap

This 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?	The 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?	Small perturbations are sometimes remediable using a robust spinfit technique. Large perturbations probably cannot be corrected.
Why does it occur?	The 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: 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. More information can be found in the document Cluster EFW Electric Field Measurements in the Polar Wind by Anders Eriksson et al.
Other notes

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http://cluter.irfu.se/ops/problems/polar_cap.html
last modified on 28-Nov-2002