ABOVE: (Left) Various current systems supported in the space environment around the Earth. (Right) The Earth's ring current as detected by the IMAGE spacecraft.

When large disturbances in the Solar Wind impact the Earth's Magnetosphere, a "magnetic storm" can develop. These storms are at least in part identified by strong enhancements in electric currents that flow through the ionosphere and magnetosphere that in turn produce magnetic field fluctuations that can be detected from the ground by networks of Geomagnetic observatories.

Throughout most of the Earth, in the mid-latitudes, the development of a magnetic storm is best indicated by Disturbance Storm Time Index (Dst). This index is simply defined as the worldwide average of deviations of the magnetic field parallel to the ground - called the H component. This change in the magnetic field results from changing electrical currents at the magnetopause - the boundary between the Earth's magnetic field and the solar wind, as well as the ring current, carried by charged particles moving around the Earth.

Large negative values of Dst indicate that we are in a strong geomagnetic storm. These magnetic changes are measured in nano-Teslas (nT). The Earth's magnetic field strength at the surface is ~50,000 nT, so a -100 nT change is a small deviation.) A Dst rating of -20 nT indicates relatively quiet conditions, while a value of -100 nT indicates a major geomagnetic storm is in progress. In general the more negative the value, the stronger the predicted storm effect. The forecasted value above uses a model that takes into account the solar wind conditions measured by the ACE spacecraft and then predicts the impact of these conditions on the Earth's magnetosphere.