The plasmasphere, also called the inner magnetosphere, is a part of Earth's magnetosphere made up of low-energy (cool) plasma. It is found above the ionosphere. The outer edge of the plasmasphere is called the plasmapause, which is marked by a large decrease in plasma density. In 1963, American scientist Don Carpenter and Soviet astronomer Konstantin Gringauz proved the existence of the plasmasphere and plasmapause by studying data from very low frequency (VLF) whistler waves. Traditionally, the plasmasphere has been viewed as a calm, cold plasma where the movement of particles is controlled only by Earth's magnetic field and, therefore, moves along with Earth's rotation.

History

The study of whistlers, sounds caused by very low frequency (VLF) radio waves, led to the discovery of the plasmasphere. Radio operators first heard whistlers in the 1890s. In 1953, British scientist Llewelyn Storey showed that lightning creates whistlers in his PhD dissertation. Around the same time, Storey suggested that whistlers meant plasma was present in Earth’s atmosphere and that plasma moved radio waves in the same direction as Earth’s magnetic field lines. From this, he proposed the plasmasphere existed, but he could not prove it for sure. In 1963, American scientist Don Carpenter and Soviet astronomer Konstantin Gringauz, working separately, proved the plasmasphere and plasmapause existed using data from the Luna 2 spacecraft and other experiments. Their work built on Storey’s earlier ideas.

In 1965, Storey and French scientist M. P. Aubry studied data from FR-1, a French satellite designed to measure VLF frequencies and the density of plasma electrons. 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. Information from the VLF receiver on OV3-3, launched on August 4, 1966, helped scientists locate the plasmapause.

In 2014, the THEMIS mission found that density changes, such as plumes or biteouts, can form in the plasmasphere. It was also discovered that the plasmasphere does not always move with Earth. The magnetosphere contains plasma at 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 Earth’s electric and magnetic fields move it. Data from the Van Allen Probes show that the plasmasphere also stops highly energetic electrons from space from reaching Earth’s surface and low Earth orbits.

• An image from the IMAGE satellite showing Earth’s plasmasphere using its Extreme Ultraviolet (EUV) imager.
• A visualization of Earth’s radiation belts, showing charged particles (blue and yellow) and the plasmapause boundary (blue-green surface).