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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 30, issue 1
Ann. Geophys., 30, 153–162, 2012
https://doi.org/10.5194/angeo-30-153-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 30, 153–162, 2012
https://doi.org/10.5194/angeo-30-153-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

  16 Jan 2012

16 Jan 2012

Non-storm irregular variation of the Dst index

S. Nakano and T. Higuchi S. Nakano and T. Higuchi
  • The Institute of Statistical Mathematics, Tachikawa, Japan

Abstract. The Dst index has a long-term variation that is not associated with magnetic storms. We estimated the long-term non-storm component of the Dst variation by removing the short-term variation related to magnetic storms. The results indicate that the variation of the non-storm component includes not only a seasonal variation but also an irregular variation. The irregular long-term variation is likely to be due to an anti-correlation with the long-term variation of solar-wind activity. In particular, a clear anti-correlation is observed between the non-storm component of Dst and the long-term variation of the solar-wind dynamic pressure. This means that in the long term, the Dst index tends to increase when the solar-wind dynamic pressure decreases. We interpret this anti-correlation as an indication that the long-term non-storm variation of Dst is influenced by the tail current variation. The long-term variation of the solar-wind dynamic pressure controls the plasma sheet thermal pressure, and the change of the plasma sheet thermal pressure would cause the non-storm tail current variation, resulting in the non-storm variation of Dst.

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