Equivalent Current Representation Research Articles

On polar daily geomagnetic variation

<p style="-qt-block-indent: 0; text-indent: 0px; margin: 0px;">The aim of this work is to investigate the nature of the daily magnetic field perturbations produced by ionospheric and magnetospheric currents at high latitudes. We analyse the hourly means of the X and Y geomagnetic field components recorded by a meridian chain of permanent geomagnetic observatories in the polar region of the Northern Hemisphere during a period of four years (1995-1998) around the solar minimum. We apply a mathematical method, known as natural orthogonal component (NOC), which is capable of characterizing the dominant modes of the geomagnetic field daily variability through a set of empirical orthogonal functions (EOFs). Using the first two modes we reconstruct a two-dimensional equivalent current representation of the ionospheric electric currents, which contribute substantially to the geomagnetic daily variations. The obtained current structures resemble the equivalent current patterns of DP2 and DP1. We characterize these currents by studying their evolution with the geomagnetic activity level and by analysing their dependence on the interplanetary magnetic field. The obtained results support the idea of a coexistence of two main processes during all analysed period although one of them, the directly driven process, represents the dominant component of the geomagnetic daily variation.</p>

The equatorial counter electrojet: Part of a worldwide current system?

The method of natural orthogonal components was applied to the ground geomagnetic data in the Central Asian sector (72–83°E), during the summer months of 1995, in an attempt to identify additional current systems that are superposed on the normal Sq current vortex and related to the equatorial counter electrojet (CEJ). The principal components, when examined in equivalent current representation, provide useful insights into the behavior of ionospheric current systems on different days. Statistical analyses performed in the present work suggest a possible relationship between the CEJ field and the noontime D variation observed at low latitudes. The results are in conformity with an earlier global simulation model.

Efficient analysis of planar scattering problems with multiple excitations using equivalent incident currents

Large planar scatterers can be analyzed either by a direct moment method scheme, leading to block Toeplitz matrices for certain geometries, or via iterative techniques. In any case the physical optics component of the solution is readily known, hence the burden of its computation should be eliminated. An inherent separation between the physical optics and the remaining “fringe current” components is achieved by the use of the equivalent current representation of the incident fields. This representation has been beneficial in the recently introduced “add‐on” scheme, as well as other methods based on modular computations. In this work it is used to provide an extra degree of efficiency to the analysis of electromagnetic scattering problems from planar structures with many incident fields. The approach extends the capability of a recently introduced method whereby a class of incident electric fields is represented by a set of basis functions. Here the equivalent incident current is expanded into a set of basis functions restricted to a limited aperture region only, facilitating the computation of the fringe current only. The response to any incident field is synthesized subsequently from the responses to these excitations. This results in a stable scheme which can efficiently treat large problems with many excitations.