The terrestrial ring current: Origin, formation, and decay

The terrestrial ring current is an electric current flowing toroidally arou nd the Earth, centered at the equatorial plane and at altitudes of similar to 10,000-60,000 km. Changes in this current are responsible for global dec reases in the Earth's surface magnetic field, which are known as geomagneti c storms. Intense geomagnetic storms have severe effects on technological s ystems, such as disturbances or even permanent damage to telecommunication and navigation satellites, telecommunication cables, and power grids. The m ain carriers of the storm ring current are positive ions, with energies fro m similar to 1 keV to a few hundred keV, which are trapped by the geomagnet ic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosph ere: The injection process involves electric fields, associated with enhanc ed magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind orig in, while geospace activity tends to increase the abundance (both absolute and relative) of O+ ions, which are of ionospheric origin. During intense m agnetic storms, the O+ abundance-increases dramatically, resulting in a rap id intensification of the ring current and an O+ dominance around storm max imum. This compositional change affects, among other processes, the decay o f the ring current through the species- and energy-dependent charge exchang e and wave-particle scattering loss. Energetic neutral atoms, products of c harge exchange, enable global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, th e effects of compositional variations in the ring current, the effects of s ubstorms on ring current growth, and the dynamics of ring current decay wit h an emphasis on the process of charge exchange and the potential for wave scattering loss.