An Empirical Relationship Between Interplanetary Conditions and Dst

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An Empirical Relationship Between Interplanetary
Conditions and Dst
Burton, R. K., R. L. McPherron and C. T. Russell,
JGR, 1975.
Journal Club ---- Winter 2007 01/17/2007
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• Dst index is a direct measure of the H-
  component of the magnetic perturbations at
  mid-latitude ground stations.
• At low to mid latitudes the H (northward)
  component of the magnetic perturbation is
  dominated by the intensity of the magnetospheric
  ring current. Dst then is proportional to the
  total energy of the drifting particles in the
  ring current.

Sudden Enhancement
Main phase
Recovery phase
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1. Objective Develop an empirical
relationship for the rate of change of Dst in
terms of the dawn-dusk electric field and the
dynamic pressure in the solar wind. 2.
Algorithm a) H Hmp Hrc (here, the tail
current is included in the Hrc ) b)
c) Determine the parameters
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Example 1

• A dynamic pressure pulse around 1200UT produced
  the sudden enhancement in Dst.
• The following postive Ey generated the main phase
  of the storm.
• The subsequent reduction in Ey initiated the
  recovery phase.

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Example 2

• The dynamic pressure enhancement produced the
  variation in Dst between 0000 and 0008UT. No
  injection due to the negateive Ey.
• Additional ring current injection retarded the
  decay of Dst between 0014 and 0019UT.
• The decay of the measured Dst was slower than
  the predicted one.

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Example 3

• Highly variable Ey and overestimated dynamic
  pressure effect possibly caused the discrepancy
  between the predicted and measured Dst.
• The choice of quiet day can affect the
  prediction and produce the deviations.

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Example 3

• The empirical model predicted lower minimum of
  the Dst than the observed one. The injection to
  the ring current F(Esw) seemed to be
  overestimated.

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Discussion

• The derived functional relationship and the
  optimum parameters depend on the state of the
  magnetosphere and its previous history. Here the
  paper assumes that this dependence is relatively
  small.
• The deviation of the predicted Dst from the
  measured Dst could be due to the computation of
  the ground Dst. It involves the issue of how to
  select the quiet days.
• The predicted sudden enhancement sometimes also
  deviate from the measured Dst. It is possibly due
  to the accuracy of the solar wind density and
  velocity measurements .
• This study only calculated the storms with the
  Dst minima gt -120 nT. For larger storms (Dst lt
  -300 nT) this model should also be able to
  predict the Dst if the ring current injection
  rate is high, i.e. strong dawn-dusk electric
  fields (high solar wind velocity and strong
  southward IMF) and sustain for longer periods.