HR: 17:45h
AN: GP34A-08    [Abstracts]
TI: Local-time stacking of geomagnetic solar daily variations
AU: * Everett, M E
EM: everett@geo.tamu.edu
AF: Texas A&M University, Dept of Geology and Geophysics, College Station, TX 77845 United States
AB: Regular geomagnetic solar daily variations, the Sq signal, is used in electromagnetic induction studies to investigate the electrical conductivity structure of Earth's upper mantle. Traditionally, Sq induction studies employ observatory magnetic data from a few quiet days in which the overprint on the regular Sq ionospheric current vortices from various irregular disturbances is minimal. Active days are typically discarded, a procedure which results in a gappy time series that is difficult to analyze. Furthermore, the quiet day selection is largely arbitrary since daily geomagnetic activity levels range continously from very quiet to very active. This fact precludes establishment of rigorous quiet/active thresholds. The average local-time variation on active days resembles typical quiet-day variations except for a multiplicative scaling factor. A multi-year stack of year-long geomagnetic time series aligned with respect to local time incorporates "regular active" days while greatly attenuating the effects of irregular geomagnetic disturbances. External and internal current systems that rise and fall in step with the sun's transit across the sky above an observatory contribute coherently to the local-time stack. All other current systems are incoherent and tend to stack out. An exact one-year-long stack preserves the seasonal variation. The resulting stacked time series contains no gaps, is dominated by regular solar daily variations, and can be used for improved Sq induction studies. Hourly geomagnetic values from Niemegk, Boulder and Mbour observatories between 1970-2002, along with the ap and F10.7 indices, are used in this study.
DE: 1515 Geomagnetic induction
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005