Solar quiet Current and Conductivity-Depth Structure of Asia Sub-Region from Magnetic Field Variation at Moderate Solar Activity

Abstract

The study used some selected magnetic field records to establish the equivalent external and internal ionospheric current system and also profile the conductivity structure beneath the Asian sub-region during the moderate solar quiet year 2012. Spherical harmonic analysis (SHA) was employed to separate the external and internal Sq current and also harmonic coefficients used to probe the conductivity values. The result showed that the month-by-month behavior of the external and internal equivalent current is stronger in local summer months (May-August) and weaker in local winter months (November-February). The current vortex appears earlier in local summer than in local winter months and disappeared in December. In each of the month, the latitudinal position of the internal Sq current vortex appeared at a considerable higher latitude relative to the external. A regression line fitted to the estimated conductivity values gives a profile that starts at 0.02 S/m at a depth of about 30 km and rose gradually to about 0.03 S/m seen at 120 km depth. Consequently the conductivity profile increases significantly to about 0.07 S/m at depth of about 400 km followed by a sharp steep increase through the transition zone and reached its peak value 0.3 S/m at depth of about 770 km. An evidence of discontinuity was observed at depth of about 370-440 km and 603-671 km. The sharp steep increase in conductivity profile at depth of about 400 km may be due to phase change of the mantle minerals from olivine to spinel structure.

Keywords: Solar quiet current, Geomagnetic field, Ionosphere, Mantle conductivity

DOI: 10.7176/JEES/15-5-05

Publication date: October 31st 2025