Quiet-time Geomagnetic Field Variations Computed from Spherical Harmonic Coefficients and Mantle Conductivity Profile over Asia Sub-region

The study used quiet magnetic field records to study the external, internal and sum of the Sq field variations during extremely quiet year 2008 in Asian sub-region. Spherical harmonic analysis (SHA) technique was used to separate the external and internal Sq field contributions. The result shows that the daytime external MSqHext, internal MSqHint, and sum MSqHTot equator-ward (pole-ward) of the Sq focus are bounded by eastward (westward) current that decreases (increases) with increasing latitudes. The MSqDext shows a noticeable absence of northward (southward) current flow during the pre-noon (afternoon) periods at LKW suggesting possible cancelation of the meridional current effect. The slight phase changes associated with variable pattern in MSqZint suggest likely electromagnetic induction effect. The coefficients of the external and internal parts of the quiet geomagnetic field of the stations and the application of Schmucker’s equivalent substitute conductivity method allowed for the determination of conductivity profile to a depth of about 700 km. The profile shows existence of weak conductivity 0.015 S/m at depth range 40 and 100 km and also 200-280 km. At greater depth, the profile shows an exponential increase indication of possible change in mineral composition bound this region. The profile also shows an evidence of discontinuity at depth 380-460 km. The conductivity profile reveal various phases of mineral composition and transformation thus indicates lateral inhomogeneous layer bound the Asian sub-region in contrast to the earlier models that reported lateral homogeneous layer Keywords: Magnetic field, Electric field, ionosphere, Solar quiet (Sq) current, Conductivity