How Dst relates to storms

The Dst index is widely used to define the occurrence, duration and magnitude of a storm (see "Magnetic Storms" for more information), although there is no single accepted definition. A temporal profile of Dst that would typically be recognised as a storm is shown below.

A typical storm in Dst

Prior to the storm, Dst varies about some level around 0 nT. The start of the storm is identified by a rapid drop in Dst over several hours. It reaches a minimum and then recovers more slowly to pre-storm levels.

The minimum Dst value reached is often used to classify the strength of a magnetic storm. A common classification (for example Loewe and Prölss) is as follows:

Weak storm	Minimum Dst below -30 nT
Moderate storm	Minimum Dst below -50 nT
Strong storm	Minimum Dst below -100 nT
Severe storm	Minimum Dst below -200 nT
Great storm	Minimum Dst below -350 nT

Based on these criteria, we have defined activity levels as shown by the coloured bar on the right hand side of the graph. Low activity is defined as Dst > -20 nT, medium activity is -20 nT < Dst < -50 nT and high activity Dst < -50 nT.

We define a magnetic storm as any interval for which Dst is less than -20 nT and during which Dst drops below -50 nT (into the high activity zone). Thus a Dst point in the green zone indicates no storm, and a storm is identified as transitions from green to amber to red, and then back though amber to green.