Decadal Variations in Geomagnetic Observatory Data from Empirical Mode Decomposition

Records of geomagnetic variability and the core flow models constructed from them have been seen to contain variations on decadal time scales. For example, the so-called 60-year signal of geomagnetic variability has historically received much attention. Furthermore, the variations found in core flow models have been interpreted as the signature of normal modes of torsional oscillations of the fluid core, an interpretation supported by comparisons with variations in length-of-day. Here we reassess the decadal variability within the observatory records using empirical mode decomposition (EMD) and compare with the periods found in the core flow models. EMD is a useful method in this context as it is capable of extracting periodic signals that are relatively long in comparison to the duration of the time series. The most prevalent periods found in the inclination and declination records from a set of 48 globally distributed observatories are (approximately) 81, 62, 30.5 and 11.5 years. The 11.5 year signal is believed to be related to the sunspot cycle, and the longer periods to processes within the core. There is some agreement between these periods and those found in core flow models; the differences may arise from the non-linear relation between core flow and geomagnetic variations, or from the 'averaging' of observatory data during the construction of the core flow models.