Mountains versus valleys: Semiannual variation of geomagnetic activity

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書誌事項

公開日
2000-02
権利情報
  • http://onlinelibrary.wiley.com/termsAndConditions#vor
DOI
  • 10.1029/1999ja900439
公開者
American Geophysical Union (AGU)

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説明

<jats:p>The semiannual variation in geomagnetic activity is generally attributed to the Russell‐McPherron effect. In that picture, enhancements of southward field <jats:italic>B<jats:sub>s</jats:sub></jats:italic>, near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the Ψ angle between the solar wind flow direction and Earth's dipole axis) that makes <jats:italic>B<jats:sub>s</jats:sub></jats:italic>, coupling less effective (by ∼25% on average) at the solstices. Thus the semiannual variation is not simply due to “mountain building” (creation of <jats:italic>B<jats:sub>s</jats:sub></jats:italic>) at the equinoxes but results primarily from “valley digging” (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the <jats:italic>am</jats:italic> index, is responsible for ∼65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time‐differentiated <jats:italic>Dst</jats:italic> index and the <jats:italic>AE</jats:italic> index.</jats:p>

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