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

A new technique for continuation of the ground magnetic field caused by ion ospheric 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 mea sured ground magnetic disturbance is expanded in terms of the ground magnet ic effect of a spatial distribution of such elementary current systems. Usi ng a matrix inversion technique, the scaling factors for each elementary cu rrent system, and therefrom the ionospheric equivalent currents are calcula ted. The technique can be applied to both global and local scales. Its adva ntages compared to the common field continuation techniques with Fourier (l ocal scale), spherical cap (local to medium scale), or spherical (global sc ale) harmonic expansions are: 1) No fixed limitation of the spectral conten t 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 mos t suitable with respect to the available measurement sites or the type of c urrent 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.