The Dst (disturbance storm time) index measures geomagnetic storm intensity by tracking how much Earth's equatorial magnetic field is depressed by the storm-time ring current. Quiet times sit near 0 nT; strong storms drive Dst below −100 nT, and the 1859 Carrington event is estimated near −900 nT.
Understanding Dst Index
Anatomy of a storm, as Dst tells it
A classic storm signature reads like a chart-recorder novel. First, often, a small positive jump — the sudden commencement, as a CME shock compresses the dayside magnetosphere and briefly strengthens the surface field. Then the main phase: hours of steep decline as southward interplanetary field drives injection after injection into the ring current, Dst plunging tens to hundreds of nanoteslas. Finally the recovery: an exponential climb back over one to several days as the trapped particles are lost to charge exchange and the current dissipates — sometimes interrupted mid-recovery by the next CME, stacking storms into the multi-day sagas that produce the deepest minima. Reading Dst traces teaches storm morphology faster than any textbook, which is why the index anchors both research and the historical record of extremes.
Dst, Kp and the scales — which gauge when
The indices answer different questions. Kp (and its linear sibling ap) asks "how disturbed is the field globally, right now?" — ideal for aurora hunters and the operational G-scale, which is defined from it. Dst asks "how much energy has this storm banked in the ring current?" — the better physics gauge of storm magnitude and duration, and the standard axis for comparing events across decades. They can disagree instructively: a brief, sharp disturbance can spike Kp while barely denting Dst, and a slow-building storm can deepen Dst impressively between Kp's three-hour samples. Researchers add finer instruments still (SYM-H, effectively a one-minute Dst), but for the question "how big was that storm, really?" the answer is customarily quoted in Dst nanoteslas.