At a height of 3-5 RE (RE =6378 km) almost all atmospheric particles are fully ionized. At this height, the motion of charged energetic particles may be approximated by the superposition of three types of motion: gyration about the main field, bounce along field lines and azimuthal drift in rings around the Earth. The drift of charged particles in the Earth's inner magnetic field produces the so-called ring-current. This current fills a doughnut-shaped volume around the equator and produces a magnetic field at the Earth's surface which reduces the strength of the surface field. In quiet conditions, the effect of ring current on the magnetic field is negligible but during magnetic storms the intensity of the ring current increases and produces significant disturbances.

The conductivity in the ionosphere is proportional to electron density which means that particle precipitation in the ionosphere leads to high conductivity. The plasma convection in the presence of Earth's magnetic field causes an electric field. Both the enhanced conductivity and the electric field cause large currents at altitudes between 90 and 150 km. Some of the consequences of these currents are joule heating of the ionospheric plasma and changes of the geomagnetic field on the ground. These changes of the geomagnetic field create the geomagnetically induced currents in the Earth and in conductors.

The three phases of a magnetic storm are

An increase of magnetic field lasting a few hours
A large magnetic depression over an interval of several hours
A slow recovery over the following few days

The initial phase is caused by the compression of the magnetosphere on the arrival of a burst of solar plasma. The main phase is due to a large increase in the ring current energy. The decay of this current will restore the magnetic field in its initial condition.

LINK: [link to large.stanford.edu]