Local time behaviour of low frequency geomagnetic field fluctuation power at low latitude

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In this paper we present a statistical study of the local time behaviour of low frequency (0.7-4.0 mHz) geomagnetic field fluctuation power at a low latitude station (corrected geomagnetic latitude 36.2°N). The analysis was conducted for two contiguous years during the ascending phase of the solar cycle. We found that the power of the horizontal east-west geomagnetic field component shows a diurnal and seasonal variation which can be related to variations of the ionospheric current system (Sq) mainly produced by dynamo-action in the ionospheric E region. The power of the horizontal north-south geomagnetic field component is higher in the local afternoon with respect to the morning; this asymmetry is more evident during time intervals characterized by high solar wind dynamic pressure.

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    ABSTRACT: We installed two electromagnetic (EM) monitoring systems in the im- mediate aftermath of the 16 October 1999 Hector Mine earthquake to search for possible continuing ultralow-frequency (ULF) EM activity due to the mainshock as well as for any precursory or coseismic EM signals that might be associated with large aftershocks. We installed the first portable monitoring system 2.5 days after the M 7.1 Hector Mine earthquake at a location 16 km southeast of the epicenter and 2 km east of the surface rupture. A second system was installed on 29 October 1999, 10 km northwest of the epicenter and within 100 m of the surface rupture. Our continuous measurements of multiple-component magnetic field, electric field, and ground motion span the low frequencies appropriate for recording possible EM sig- nals generated at seismogenic depths and were carried out during 3 months following the mainshock. Continuous magnetic-field measurements at observatory EM stations operating in California are used as remote-reference sites to remove global atmo- spheric signals, which helps isolate local terrestrial sources of interest. Our analysis of preseismic ULF-EM variations, the coseismic response, the 2-month-long magnetic-field power spectra, and electric-field polarization shows no anomalous behavior clearly associated with seismic activity.
    Bulletin of the Seismological Society of America 05/2002; 92(4):1513-1524. DOI:10.1785/0120000919 · 2.32 Impact Factor