The Dst (Disturbance storm-time) index represents the axially symmetric disturbance of the magnetic field at the dipole equator on the Earth's surface [Sugiura, 1964]. Dst values above -20 nT correspond to quiet behaviour of the field, values below -50 nT indicate a moderate disturbance in the field. In extreme cases, Dst can drop to below -400 nT.
Dst is measured by a worldwide network of instruments, which send their data to the World Data Centre for Geomagnetism, Kyoto. Because preparing the Dst index for scientific study is a complex process, several versions of Dst are released, from quick-look Dst which is released between 12 and 36 hours after the measurements are made, to the final Dst index, which arrives several years afterwards.
An example of quick look Dst for this month and last month.