Abstract
The C-responses of selected observatories are determined for 6 frequencies (1 cpd-6 cpd) using the Z: Y-method: Horizontal components of the magnetic variations are expanded into series of spherical harmonics to achieve a global representation of the unseparated magnetic potential. The C-response of each site is evaluated by comparison with the locally observed Z-variation.
The spherical harmonic analysis is accomplished for each day except D-days separately and coefficients with n=1, ..., 10 and m=-n, ..., n (120 coefficients) are determined. A damping constant α2-n8 is added to the diagonal elements of the normal equations to avoid oscillations (after Marquardt). Mean values of the C-responses are estimated for each observatory using robust methods.
The results using 54 months of world-wide data (≈100 observatories) are consistent for different data sets and adjacent observatories but show a clear regional trend in Europe with decreasing resistivity from the West to the East.
