Energetic neutral atom image of Earth's ring current during the recovery phase of a geomagnetic storm. (larger version and caption)
The ring current is one of the major current systems in the Earth's magnetosphere. It circles the Earth in the equatorial plane and is generated by the longitudinal drift of energetic (10 to 200 keV) charged particles trapped on field lines between L ~ 2 and 7. During geomagnetic storms, ring current particle fluxes are dramatically increased, with the peak enhancements occurring in the inner ring current (at L < 4). The quiet-time ring current consists predominantly of H+, while the storm-time ring current also contains a significant component of ionospheric O+, whose contribution to ring current energy density may even exceed that of H+ for brief periods near the maximum of particularly intense storms.

The formation of the storm-time ring current has been attributed to two different processes: (i) the injection of plasma into the inner magnetosphere during the expansion phase of magnetospheric substorms and (ii) increased convective transport of charged particles from the nightside plasma sheet deep (L < 4) into the inner magnetosphere as a result of an intensification of the Earth's dawn-dusk convection electric field during extended periods of strong southward IMF. The present understanding of ring current formation tends to favor the enhanced convection model over the substorm plasma injection model; however, it is recognized that substorms, while not the primary driver, nonetheless play a significant role in the growth of the storm-time ring current (e. g., by energizing ions in the near- Earth plasma sheet prior to their transport into the ring current).

The storm-time growth of the ring current lasts from 3 to 12 hours and constitutes the "main phase" of a magnetic storm. Following this main phase, the ring current begins to decay, returning to its pre-storm state in two to three days. (Full recovery can require as long as a month in the case of major geomagnetic storms.) During the storm recovery phase, particle transport into the ring current slows, allowing various loss processes to reduce ring current particle fluxes to their quiet-time level. The primary loss process during both the main and recovery phases is charge exchange with neutral hydrogen atoms in the geocorona. A second loss process, affecting principally low-energy ring current ions, involves Coulomb collisions with the thermal plasma of the plasmasphere. The third process thought to contribute to ring current decay is the precipitative loss of ring current particles into the atmosphere as a result of wave-particle interactions. The role of this loss process in the evolution of the ring current is still not well understood and is the subject of ongoing research.

The growth and recovery of the ring current are indicated by changes in the Dst (disturbance storm time) index, the geomagnetic index that serves as the standard measure of ring current activity.