The disturbance storm time index, also known as the Dst index or Kyoto Dst index, is a measure of the strength of the Earth's ring current. The ring current around Earth produces a magnetic field that is directly opposite Earth's magnetic field; that is, if the difference between solar electrons and protons gets higher, then Earth's magnetic field becomes weaker. A negative Dst value means that Earth's magnetic field is weakened. This is particularly the case during solar storms. Its units are typically measured in nT (nanoteslas).
See also K-index
References
External links The Dst index homepage provided by Kyoto University Dst at NOAA/NGDC Dst as part of SWENET Latest Alerts Archived 2012-02-09 at the Wayback Machine (on ESA's Space Weather Portal)