ESA Science & Technology11-Jul-2005 09:40:55

Plasma Regions

The auroral zone

The auroral zone is a ring of light emission created by the precipitation of particles in the atmosphere and centred around the magnetic pole. The cusp and boundary layers on the dayside and the plasma sheet and plasma-sheet boundary layer on the nightside are the sources of these precipitations. Transient plasma flows and particle participation into the low-altitude cusp are of great interest, as they appear to be produced by phenomena occurring on the magnetopause, like flux transfer events or solar wind dynamic pressure variations. The spatial scales and time variations of these transients are very important to distinguish between these two mechanisms. The four Cluster  II spacecraft will cross the cusp between 4 and 6 Earth radii rather like a string of pearls, at intervals ranging from a few minutes to up to 40 minutes, allowing the time variations of the transient events to be studied.

The ionosphere is now believed to be an important source of plasma for the magnetosphere and in particular for the magnetotail. The polar wind, the cleft ion fountain, and the nightside auroral zone contribute to the filling of the magnetosphere with plasma. However, their exact location may have been underestimated due to the limited measurements at low energy (less than 10  eV). Cluster  II should help to refine these results with ion measurements (with distinction of mass) down to around 2  eV.

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