There are more and more high-quality global geomagnetic data sets available with dense temporal and spatial resolution or covering long time spans. In addition to the progress of outer space field research and crustal studies these data sets allow for more precise modelling in the deep earth interior if they are coupled with quantities related to dynamics and geodynamo calculations. Thus, it is desirable to know the temporally magnetic field in the zone around the core-mantle boundary (CMB) with high resolution in space and time. For this, an appropriate approach, the non-harmonic downward continuation method, has been implemented which is based on the solution of an inverse ill-posed initial-boundary value problem. It calculates the field at a lower boundary in the deep earth interior with data (Gauss coefficients) given at the earth surface and assuming any radially dependent mantle conductivity model. Here with some examples, we demonstrate the possibilities of this method and show its sensitivities. We compare the resulting fields with those basing on a non-conducting mantle. The main differences are found in the amplitudes and phase shifts of temporally varying processes. This is shown in detail for all three magnetic field components at the CMB by means of simulated oscillations. In addition, the dependence on the assumed conductivity or conductance is analysed, especially the influence of conductivity jumps possibly existing in the vicinity above the CMB. Clear phase shifts can be observed if rapidly proceeding temporal trend changes (with its high frequency character)are mapped from the earth surface onto the CMB region where this phenomenon can be seen at an earlier stage and closer to the causing processes.

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Ballani, L.; Stromeyer, D.; Greiner-Mai, H.; Hagedoorn, J. (2007): The non-harmonic downward continuation method in deep earth magnetic field studies.. 24th IUGG 2007 (Perugia, Italy, 2007).