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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.)
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