Decomposing the geomagnetic field: oscillation modes and characteristics

Using the COV-OBS.x1 (Gillet et al., 2015) main geomagnetic field model, covering the time span 1840–2020, respectively IGRF-13 (1900-2020), we decomposed the geomagnetic field at Earth’s surface in oscillation modes by means of empirical orthogonal functions (EOF) as well into a long term and a cyclic component using HP filtering (Hodrick and Prescott, 1997). Further, the long term component is filtered using a Butterworth filter (1930) with different cut-off periods in order to obtain oscillation at inter-centennial (> 100 years) and sub-centennial (60-90 years) timescales. The EOF analysis shows that the first three oscillation modes are characterized by periodicities of >100 years while modes 4 and 5 are characterized by dominant periodicities of 70-90 year. Although the variance of the modes 4 and 5 is rather small compared to that of the first three modes, these two modes are responsible for the detailed structure of the geomagnetic field. A comparison between the results of both methods is done as well.