Evidence for a Dominant Russell-McPherron/Rosenberg-Coleman Origin of the Semiannual Variation of Geomagnetic Activity in 1954 and 1996

Details Evidence for a Dominant Russell-McPherron/Rosenberg-Coleman Origin of the Semiannual Variation of Geomagnetic Activity in 1954 and 1996 Evidence for a Dominant Russell-McPherron/Rosenberg-Coleman Origin of the Semiannual Variation of Geomagnetic Activity in 1954 and 1996

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm72b0614c&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #SM72B-0614

Physics

1555 Time Variations: Diurnal To Secular, 2784 Solar Wind/Magnetosphere Interactions, 2788 Storms And Substorms

Scientific paper

Occasionally, the semiannual variation of geomagnetic activity is so pronounced that one can readily identify it in daily averages of the aa index during the year. The solar minimum years of 1954 and 1996 were two such intervals. Using solar eclipse data and the Svalgaard polarity index for 1954 and solar magnetic field and solar wind data for 1996, we show that the six-month wave in geomagnetic activity during these years was primarily due to a flattened current sheet resulting in a strong Rosenberg-Coleman effect (an axial polarity effect), which in turn produced a strong Russell-McPherron response in aa. When we normalize the aa data for these years for the equinoctial effect (based on the angle between the solar wind flow direction and Earth's dipole), we remove approximately 30% of the amplitude of the semiannual variation, implying a dominant axial/Russell-McPherron origin. When we perform this normalization for the entire 1868-1998 aa data set, we remove 75% of the six-month wave, indicating that, in general, the equinoctial effect is primarily responsible for the semiannual variation of geomagnetic activity.

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