The plasmasphere, also called the inner magnetosphere, is a part of Earth's magnetosphere that contains low-energy plasma. It is found above the ionosphere. The outer edge of the plasmasphere is called the plasmapause, which marks a sharp decrease in the number of plasma particles. In 1963, American scientist Don Carpenter and Soviet astronomer Konstantin Gringauz proved the plasmasphere and plasmapause exist by studying data from very low frequency (VLF) whistler waves. Usually, the plasmasphere is considered a stable, cold plasma where the movement of particles is controlled mainly by Earth's magnetic field, causing it to rotate along with Earth.

History

The discovery of the plasmasphere came from the scientific study of whistlers, natural events caused by very low frequency (VLF) radio waves. Radio operators first noticed whistlers in the 1890s. In 1953, British scientist Llewelyn Robert Owen Storey showed that lightning created whistlers in his PhD dissertation. Around the same time, Storey suggested that the presence of whistlers meant plasma existed in Earth's atmosphere and that plasma moved radio waves in the same direction as Earth's magnetic field lines. From this, he concluded that the plasmasphere might exist, but he could not prove it for sure. In 1963, American scientist Don Carpenter and Soviet astronomer Konstantin Gringauz independently proved the plasmasphere and plasmapause existed, using Storey’s earlier ideas. Gringauz used data from the Luna 2 spacecraft for his work.

In 1965, Storey and French scientist M. P. Aubry studied data from FR-1, a French satellite designed to measure VLF frequencies and the electron density of plasma. Their analysis of FR-1’s data supported their theories: VLF waves in the ionosphere sometimes passed through a thin layer of plasma into the magnetosphere, moving perpendicular to Earth’s magnetic field lines. During the 1970s, Storey continued studying VLF waves using data from FR-1. Data from the VLF receiver on OV3-3, launched on August 4, 1966, helped scientists locate the plasmapause.

In 2014, observations from the THEMIS mission showed that density irregularities, such as plumes or biteouts, can form in the plasmasphere. It was also found that the plasmasphere does not always move with Earth. The magnetosphere’s plasma has many different temperatures and concentrations. The coldest plasma is usually found in the plasmasphere. However, plasma from the plasmasphere can be found throughout the magnetosphere because it is moved by Earth’s electric and magnetic fields. Data from the twin Van Allen Probes showed that the plasmasphere also stops highly energetic ultrarelativistic electrons from space, such as those from the sun or distant cosmic sources, from reaching Earth’s surface or low Earth orbits.