Local time stacking of geomagnetic solar daily variations
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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 continuously from very quiet to very active. This fact precludes the 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 multiyear stack of yearlong 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 1-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 and comprehensive geomagnetic field modeling. Hourly geomagnetic values from Niemegk, Boulder, Mbour, and Meanook observatories between 1970 and 2002, along with the ap index, are used in this study. Copyright 2006 by the American Geophysical Union.