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Influence of the geomagnetic field, special case of the polar orbits

On high altitude orbits, e.g. the geosynchronous one, the influence of the geomagnetic field is, in general, negligible. As shown below, the case of low altitude polar orbits is different.

An example is provided by the scientific Swedish satellite Viking launched on 22 february, 1986, into a polar orbit by an Ariane rocket. Its scientific mission was the study of auroral and magnetospheric physics, particulary at medium altitudes and on high-latitude field lines. The plasma instruments included two Langmuir's probes at the extremities of two 40 meters wire booms in the plan of spin.

On the Langmuir probe signal (shown Figure 2) a periodic oscillation of the current was observed with twice the spin frequency. When the Viking satellite crossed the auroral arc (between 2034:45 and 2035:20), the oscillations had a maximum when the probe-hub axis was parallel to the magnetic field line. The phase of the oscillation indicated that this was not due to the variation of the current of photoelectrons emission with the orientation of the satellite with respect to the sun ([#!DOCXVIII!#]). So these variations was interpreted by the confinement of the photoelectrons by the magnetic field, as in a "magnetic bottle" ([#!DOCXIII!#] and [#!DOCXVIII!#]), reaching the probe when the boom is parallel to the magnetic field line and hereby increasing the current.

Figure: Example of oscillations of the current of the probe on the Swedish Satellite Viking, extract of [#!DOCXIII!#].
\mbox{\epsffile{mesviking2.ps} }\par\end{center}\end{figure}

However, another effect is expected, when the probe-boom system is parallel to the field line. The probe is in a kind of magnetic wake of the hub and its current could be decreased by the fact that the particles collected before by the hub cannot reach the probe. This effect is apparently at work at the edge of the auroral arc where the minima occur when the antenna is close to the parallel of the magnetic field. The final effect on the probe current is the sum of these two phenomenon. If the above interpretation is correct, the influence of the magnetic field depends of the angle of the boom with respect to the direction of the magnetic field and the induced perturbations are minimum at right angle.

In the following, the magnitude of these two effects compared to the ideal case of a non-perturbed current is investigated for the case when the boom is parallel to the magnetic field.


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