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.
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
|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.