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The Earth's plasmasphere is an inner part of the magnetosphere. It is a doughnut-shaped region of low energy charged particles (cold plasma) centred around the planet's equator and rotating along with it. It forms a plasma cloud that enshrouds the Earth. Its toroidal shape is determined by the magnetic field of Earth.

The plasmasphere is located above the ionosphere and extends outwards, with the outer boundary varying (depending on geomagnetic conditions). The size of the plasmasphere changes, depending on the solar windmagnetosphere interaction.

The outer boundary of the plasmasphere is known as the plasmapause.

Plasmasphere, before and after storm (Image credits Jerry Goldstein)
These three panels show how the relative locations of the outer boundary of the Earth's plasmasphere, the plasmapause, (shown in blue) and the van Allen belts (shown in red) change according to geomagnetic conditions. Credits ESA - C. Carreau
Electron density in the plasmasphere. Image credits Pierrard and Stegen, 2008.