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Cliver et al. 2000
Cliver, E.W., Kamide, Y. and Ling, A.G. (2000). Mountains versus valleys: Semiannual variation of geomagnetic activity. Journal of Geophysical Research 105: doi: 10.1029/1999JA900439. issn: 0148-0227.
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 &psgr; 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. ¿ 2000 American Geophysical Union 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 &psgr; 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. ¿ 2000 American Geophysical Union
BACKGROUND DATA FILES

Abstract

Holes in the Valleys: Diurnal Variation of Am
Relative Importance of RM & Equinoctial Mechanisms
Relative Importance of RM & Equinoctial Mechanisms
Imprint of ? On the DST & AE Data Sets

Keywords
Geomagnetism and Paleomagnetism, Rapid time variations, Ionosphere, Current systems, Magnetospheric Physics, Magnetospheric configuration and dynamics, Magnetospheric Physics, Solar wind/magnetosphere interactions
Journal
Journal of Geophysical Research
http://www.agu.org/journals/jb/
Publisher
American Geophysical Union
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