ESA Science & Technology11-Jul-2005 09:40:45

Plasma Regions

Plasma Sheet Boundary Layer

The plasma-sheet boundary layer is often the most active plasma region of the magnetotail. Ion beams are observed coming from and toward the Earth. Multi-point measurements are essential to derive the exact origin of these beams and to learn more about the generating mechanism.

Magnetic-field-aligned currents flowing toward and away from the Earth are also observed in this region. The Fluxgate Magnetometer, FGM, will be able, for the first time, to calculate the current flowing in the vicinity of the spacecraft without having to make assumptions about the shape and orientation of the current sheet.

In the centre of the plasma sheet lies the neutral sheet, where the magnetic field is weak. The neutral sheet seems to be an ideal site for magnetic reconnection, which accelerates ions towards the Earth and downstream.

The central plasma sheet is the location of the cross-tail current flowing from dawn to dusk. Sometimes this current is disrupted and part of it is directed toward the ionosphere. This disruption begins in the near-Earth tail and then propagates further downstream at high speed. These disruptions also seem to be associated with substorms, one of the most intriguing phenomena in the magnetosphere. Cluster  II will be able to sense remotely the beginning of the disruption when the spacecraft are in the lobes and then observe its consequences further downstream in the plasma sheet.

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