Ionospheric disturbance magnetic field continuation from the ground to the ionosphere using spherical elementary current systems

A new technique for continuation of the ground magnetic field caused by ionospheric currents to the ionosphere in spherical geometry is presented that makes use of elementary ionospheric current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary current systems. Using a matrix inversion technique, the scaling factors for each elementary current system, and therefrom the ionospheric equivalent currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.