ESA Science & Technology11-Jul-2005 09:32:55

Plasma Regions

The Magnetotail

Diagram of the Cluster II orbit at midsummer and midwinter

The magnetotail is characterised by magnetic field lines stretched by the solar wind flow in the anti-sunward direction. The outer region consists of two magnetail lobes and the inner region of the plasma-sheet boundary layer and the central plasma sheet. The lobes are large reservoirs of magnetic energy which contain less than one particle per cubic centimetre. Because this very tenuous plasma cannot equilibrate the flow of photo-electrons emitted by the surface, a passive spacecraft would charge to a high positive potential (up to 30  V) in this region. The effect of this potential on low-energy-particle measurement is dramatic, since the electron measurements are saturated by photo-electrons coming back to the spacecraft and ions are repelled by the potential. The technique of potential control by ion emitters has already been used on Geotail and has proved to be very valuable, particularly in the far tail.

The Cluster  II potential will be controlled by ASPOC, the Active Spacecraft Control instrument, allowing measurement of the full distribution function of the ions and electrons down to around 2  eV in the lobes and the polar cap. These measurements will determine the role of the lobes as a transit region for particles originating from the ionosphere and the solar wind.

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