Mountains versus valleys: Semiannual variation of geomagnetic activity

Details Mountains versus valleys: Semiannual variation of geomagnetic activity Mountains versus valleys: Semiannual variation of geomagnetic activity

Feb 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000jgr...105.2413c&link_type=abstract

Journal of Geophysical Research, Volume 105, Issue A2, p. 2413-2424

Physics

82

Geomagnetism And Paleomagnetism: Rapid Time Variations, Ionosphere: Current Systems, Magnetospheric Physics: Magnetospheric Configuration And Dynamics, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions

Scientific paper

The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field Bs 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 Bs coupling less effective (by ~25% on average) at the solstices. Thus the semiannual variation is not simply due to ``mountain building'' (creation of Bs) 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 am 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-differential Dst index and the AE index.

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