Example of plot from Oct. 2003

The Dst index is useful for identifying geomagnetic storms. It represents the envelop of perturbations observed in equatorial ground magnetograms.

Most storms begin with an abrupt increase in the solar wind dynamic pressure which compresses the magnetospheric magnetic field and results in a brief (several hour) enhancement in the Dst index above mean values (here set to 0 nanoTeslas).

If the interplanetary magnetic field remains strongly southward for a period of hours, the ring current of energetic particles encircling the Earth strengthens, and the corresponding magnetic field perturbations on the Earth's surface cause the Dst index to fall rapidly. As the ring current decays, the Dst index gradually returns to its mean value. The abrupt increase in Dst, decrease, and subsequent return to quiet times values represent the three phases of a geomagnetic storm.

Equatorial Dst from WDC-C2 Kyoto Low Latitude Magnetic Field Measurements