The Dst or disturbance storm time index is a measure of geomagnetic activity used to assess the severity of magnetic storms. It is expressed in nanoteslas and is based on the average value of the horizontal component of the Earth's magnetic field measured hourly at four near-equatorial geomagnetic observatories. Use of the Dst as an index of storm strength is possible because the strength of the surface magnetic field at low latitudes is inversely proportional to the energy content of the ring current, which increases during geomagnetic storms. In the case of a classic magnetic storm, the Dst shows a sudden rise, corresponding to the storm sudden commencement, and then decreases sharply as the ring current intensifies. Once the IMF turns northward again and the ring current begins to recover, the Dst begins a slow rise back to its quiet time level. The relationship of inverse proportionality between the horizontal component of the magnetic field and the energy content of the ring current is known as the Dessler-Parker-Sckopke relation. Other currents contribute to the Dst as well, most importantly the magnetopause current. The Dst index is corrected to remove the contribution of this current as well as that of the quiet-time ring current.

Dst Index for a Major Geomagnetic Storm

The plot above shows the decrease in the Dst index (solid line) and the increase in the energy content of the ring current between L = 3 and 5 (crosses) observed during a major geomagnetic storm that occurred in February 1986. Following a slow two-day build-up, the storm reached maximum intensity early on February 9, by which time the energy content of the ring current had grown to nearly sixteen times its quiet-time value and the Dst had dropped to -312 nT. The sharp rise in ring current energy content during the latter part of February 8 is due to the strong injection into the ring current of ionospheric ions; the rapid initial recovery on February 9 is due to the rapid loss of these same ions. Full recovery of the ring current to its quiet-time state required over a month.

(Source: Hamilton, D. C., et al., Ring current development during the great geomagnetic storm of February 1986, J. Geophys. Res., 93, 14343, 1988.)