Ring current

spaceweb@oulu.fi - last update: 26 November 1998, 2300 UT (RR)

Introduction

The ring current (e.g., Frank, 1967) consists of geomagnetically trapped 10 - 200 keV ions (mainly H⁺, He⁺, and O⁺) and electrons that drift azimuthally around the Earth at radial distances of about 2-7 Re (Earth radii), overlapping the radiation belt region. The drift is a combined curvature and gradient drift which is eastward for electrons and westward for ions, i.e., the direction of the current is westward.

The global strength of the ring current can be monitored by ground-based magnetometers at middle or equatorial latitudes because of its diamagnetic effect (it decreases the intensity of the Earth's magnetic field). The Dst index calculated from the observations gives an estimate of the total energy content of the particles forming the ring current. This index can be used to monitor the development of geomagnetic storm (see also geomagnetic activity), as the main signature (actually definition) of the storm main phase is the build-up of an enhanced ring current.

Ring current growth

The main enhancement of the storm-time ring current occurs at distances L < 4. Two processes have been suggested to explain this enhancement: 1) substorm particle injections and 2) transport and energization of plasma sheet particles by enhanced convection electric field. The current view is that the latter process is more capable of delivering particles as deep as L < 4, especially if the electric field fluctuates (Lyons and Schulz, 1989). Note that O⁺ dominates over H⁺ near the storm's maximum phase (Hamilton et al., 1988).

Ring current decay

Quite another topic is the decay of the ring current after a storm, since processes like charge exchange or Coulomb collision seem to be too slow. That the losses due to electromagnetic ion cyclotron (EMIC) waves might play a role here was first suggested by Cornwall et al. (1970).

References

Cornwall, J. M., F. V. Coroniti, and R. M. Thorne, Turbulent loss of ring current protons, J. Geophys. Res., 75, 4699, 1970.
Frank, L. A., On the extraterrestrial ring current during geomagnetic storms, J. Geophys. Res, 72, 3753-3768, 1967.
Hamilton, D. C., G. Gloeckler, F. M. Ipavich, W. Stüdemann, B. Wilken, and G. Kremser, Ring current development during the great geomagnetic storm of February 1986, J. Geophys. Res, 93, 14343-14355, 1988.
Lyons, L. R. and M. Schulz, Access of energetic particles to storm time ring current through enhanced radial diffusion, J. Geophys. Res, 94, 5491, 1989.