Dst index

The Dst is a geomagnetic index which monitors the world wide magnetic storm level and, for this reason, it has long been used as an indirect measure of the ring current.
This hourly index is obtained from the average of H component disturbances of the geomagnetic field from four low-latitude magnetic observatories (Honolulu, San Juan, Hermanus and Kakioka).
It is important to note, however, that the Dst index is actually a measurement of the longitudinally averaged ground perturbation at low-latitude magnetometer stations and thus measures the effects of many terrestrial and magnetospheric current systems indiscriminately. Nowadays, it is known that the Dst index is indeed influenced by the symmetric ring current, the partial (asymmetric) ring current, the near-tail current, the dayside magnetopause currents and the field-aligned currents (FACs) that couple the magnetosphere to the high-latitude ionosphere and magnetopause currents.

 Map of the geomagnetic observatories used for the evaluation of the Dst index.

  
However, if we use the Dst index to monitor the world wide magnetic storm level the negative Dst values will indicate a magnetic storm in progress. According to the different values assumed by Dst index, we may classify the geomagnetic storms in the following way:

The Dst index is prepared by the World Data Center for Geomagnetism at Kyoto University in Japan and is available from 1957 to the present at the following address:
http://www.ngdc.noaa.gov/stp/GEOMAG/dst.html

In the figure below it is reported the trend of the X magnetic field component recorded at L’Aquila observatory from July, 15 2000 to July, 17 2000 (Bastille event). To understand how the Dst index describes these type of events, we have reported the values of the Dst index for the same period.

X component of the geomagnetic field measured at L’Aquila observatory (in blue)and Dst index (in green) during the “Bastille event”.