plasmasphere

The plasmasphere is a region of cold, dense plasma within Earth's magnetosphere that co‑rotates with the planet. It forms a torus around Earth and extends outward from just above the ionosphere to several Earth radii (R_E). The outer boundary, where plasma density drops sharply, is called the plasmapause. Under quiet geomagnetic conditions, the plasmasphere typically spans roughly 1 to 4 R_E, but geomagnetic activity can compress it so that the boundary lies closer to Earth.

Composition and properties: The plasmasphere consists mainly of hydrogen ions (H+) and helium ions (He+), with electrons accompanying them. Densities are highest near the inner edge and decline toward the plasmapause, while temperatures are relatively low, characteristic of a cold plasma compared with the surrounding magnetosphere.

Formation and dynamics: Plasma in the plasmasphere is largely supplied by ionospheric outflow and is carried along magnetic field lines to fill the inner magnetosphere. Because it is bound to Earth's rotation and magnetic field, the plasma co‑rotates with the planet. Geomagnetic activity driven by the solar wind can erode the plasmasphere, shrinking the plasmapause and altering densities; during quieter times it gradually refills from the ionosphere.

Boundary and observational aspects: The plasmapause represents a sharp boundary between dense, corotating plasmasphere and the less dense outer magnetosphere. Its location changes with geomagnetic conditions and can be inferred from plasma density measurements and radio wave propagation studies, including analysis of whistler-mode waves.

Significance: The plasmasphere affects radio communications and navigation through its impact on wave propagation and refractive properties, and it interacts with radiation belts by modifying wave–particle interactions. It is studied using spacecraft measurements and ground‑ and space-based plasma diagnostics.