Modelling European magnetic repeat station and survey data by SCHA in search of time-varying anomalies

Abstract

The changing geomagnetic field interacting with conductivity anomalies in the Earth’s lithosphere should lead to time-varying magnetic anomalies, which would appear as anomalies in the observed geomagnetic secular variation. We studied 50 years of spatially dense magnetic repeat station data from Europe in search for such possible secular variation anomalies. The available data are highly non-uniform in space and time, which makes modelling difficult. Further, the accuracy of much of the data is only of the same order as the expected signal. Modelling the data by spherical cap harmonic analysis (SCHA) showed satisfactory results for low spatial indices of the harmonics. Large-scale regional secular variation can thus, be represented quite well. However, including harmonics with higher spatial index to investigate possible shorter-wavelength features or, equivalently, modelling residuals of the repeat station secular variation data after subtracting a normal secular variation field, led to numerically unstable results. The main reason for this is thought to be the relatively large data errors. A definite answer as to whether secular variation anomalies exist or not cannot yet be given. Simultaneous modelling of the data in space and time might give better results. Of greater importance, though, is a more careful elimination of external field variations from repeat station data to reduce the data errors.