Local time stacking of geomagnetic solar daily variations

Abstract

[1] 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.

An inspection of the long-term behaviour of the range of the daily geomagnetic field variation from comprehensive modelling

“…Le Mouë l et al, 2005) showed that long-term trends are similar for different choices of the days. This fact has been especially investigated by Everett (2006), who concluded that the average local time variation on active days resembles typical quiet day variations except for a multiplicative scale factor. The monthly mean range was therefore computed for each observatory as the average of the amplitude of the daily diurnal variation for the corresponding five quietest days.…”

Sq effect on the electromagnetic response functions in the period range between 104 and 105 s

“…Periods separate from the tidal and Sq periods are usually employed for the estimation to avoid not only the effects of the line spectra but also the side lobes of the line spectra that might exist due to seasonal variations of the Sq field (e.g. Schmucker 1999; Everett 2006).…”

“…Periods separate from the tidal and Sq periods are usually employed for the estimation to avoid not only the effects of the line spectra but also the side lobes of the line spectra that might exist due to seasonal variations of the Sq field (e.g. Schmucker 1999;Everett 2006). Although the periods used to estimate the response functions were carefully chosen to be separate from the Sq and tidal periods, the estimated response functions show a characteristic feature if the periods are in the ME band, as it is presented in the next section.…”

Anomaly of the geomagnetic Sq variation in Japan: effect from 3-D subterranean structure or the ocean effect?

“…The selection of suitable days follows the quietness definition introduced by Schmucker (1999). We understand that this definition involves several problems (Everett 2006) which however are not very serious in the present case of answering Rikitake's question. It is based on a fixed upper limit for a specified sum of 3‐hr ap indices.…”