ESA Science & Technology11-Jul-2005 14:25:06

Plasma Regions

The Magnetopause

The magnetopause is the thin plasma layer that separates the solar wind magnetic field from the Earth's magnetic field. At this location the plasma pressure of the solar wind is in equilibrium with the magnetic pressure inside the magnetosphere. Due to a continuous variation of the solar wind pressure, this boundary moves continuously. Cluster  II will help to characterise this motion. Although the magnetopause is usually considered as an impenetrable boundary, some plasma from the solar wind can enter the magnetosphere.

Various processes have been proposed to account for this penetration of plasma:

  1. Pressure pulses - when the solar wind dynamic pressure suddenly increases, leading to an indentation of the magnetopause
  2. Reconnection between the interplanetary magnetic field and the Earth's magnetic field
  3. Viscous interaction
  4. Impulsive penetration where plasma filaments, which have a higher momentum than the surrounding solar wind plasma, hit and possibly penetrate through the magnetosphere

Cluster  II (in particular EDI and EFW) will measure the three-dimensional size and motion of the structures observed at the magnetopause and will also be able to determine the local geometry of the magnetopause, which will make it possible to distinguish between these mechanisms.

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