Separation of the geomagnetic variation field on the ground into external and internal parts using the spherical elementary current system method

Abstract

Traditionally the separation of the ground geomagnetic field variations into external and internal parts is carried out by applying methods using harmonic functions. However, these methods may require a separate field interpolation and extrapolation, can be computationally slow, and require a minimum wavelength to be specified to which the spatial resolution is limited globally. A novel method that utilizes elementary current systems can overcome these shortcomings. The basis is the fact that inside a domain free of current flow, the magnetic field can be continued to any selected plane in terms of equivalent currents. Two layers of equivalent currents, each composed of superposition of spherical elementary systems, are placed to reproduce the ground magnetic field: One above the surface of the Earth representing the field of ionospheric origin, and one below it representing the field caused by induced currents in the Earth. The method can be applied for single time steps and the solution of the associated underdetermined linear system is found to be fast and reliable when using singular value decomposition. The applicability of the method is evaluated using synthetic magnetic data computed from different ionospheric current models and associated image currents placed below the surface of the Earth. Following these tests, the method is applied to the measurements of Baltic Electromagnetic Array Research (BEAR) (June–July 1998). External and internal components of geomagnetic variations were computed for the entire measurement period. Also the adequacy of the sparser IMAGE magnetometer network for the full field separation was tested.