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This image is a summary of the main features of the plasmasphere: the plasmapause, main body of the plasmasphere, dusk-bulge region and detached plasma regions outside the main body of the plasmasphere
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The Earth's Plasmasphere

The plasmasphere is a torus-shaped region within the Earth's magnetosphere. The Earth's plasmasphere is made of just that - plasma. It has a very sharp edge called the plasmapause at equatorial distances of 4-6 Earth radii. The plasmapause, where densities drop by a factor of 10-100 in a relatively short distance, was discovered in 1963.

The plasmasphere is essentially an extension of the ionosphere to high altitudes. Inside of the plasmapause, geomagnetic field lines rotate with the Earth. Plasma, flowing up from the ionosphere, is trapped on these corotating field lines and builds up to high densities. Outside the plasmapause, magnetic field lines are unable to corotate because they are influenced strongly by electric fields of solar wind origin. They convect to the magnetopause boundary on the day side of the Earth, sweeping the ionospheric plasma out of the magnetosphere and forming the sharp plasmasphere boundary. The plasmasphere bulges out to greater distances on the dusk side of the Earth.

The plasmasphere is composed mostly of hydrogen ions. The plasma density ranges from 104 cm-3 just above the topside ionosphere to 102 cm-3 within and 10 cm-3 just exterior to the plasmapause. The base of the plasmasphere is taken as the altitude (~ 1000 kilometers) at which protons replace oxygen as the dominant species in the ionospheric plasma. The ion temperature in the plasmasphere is generally between 0.5 eV and several eV (1 eV = 11,700 K).

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