The magnetopause is the border of the magnetosphere, a thin plasma layer that separates the magnetic field of the solar wind 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 is constantly in motion. Observations with the Cluster flotilla of spacecraft have helped to characterise this motion.
A three-dimensional cut-away view of Earth's magnetic bubble called the magnetosphere
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:
- Pressure pulses - when the solar wind dynamic pressure suddenly increases, leading to an indentation of the magnetopause
- Reconnection between the interplanetary magnetic field and the Earth's magnetic field
- Kelvin-Helmholtz vortices (see Featureed highlight below)
- Impulsive penetration where plasma filaments, which have a higher momentum than the surrounding solar wind plasma, impact and possibly penetrate the magnetopause.
Cluster measures the three-dimensional size and motion of the structures observed at the magnetopause and is also able to determine the local geometry of the magnetopause, which makes it possible to distinguish between these mechanisms.
Last Update: 29 November 2010