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 wind – magnetosphere interaction.
The outer boundary of the plasmasphere is known as the plasmapause.
The relations between the position of the plasmapause and the position of the radiation belt boundaries have been studied using the 4 Cluster satellites. For instance, during high geomagnetic activity time periods, the plasmapause is located closer to the inner boundary of the outer radiation belt.
Scientists have developed a 3D dynamic model of the plasmasphere that provides the number density of the particles and the position of the plasmapause as a function of time. Plasmaspheric density can also be determined from ground-based measurements, for instance from whistler electromagnetic waves recorded with a VLF (very low frequency) antenna.